An Lmx1b-miR135a2 regulatory circuit modulates Wnt1/Wnt signaling and determines the size of the midbrain dopaminergic progenitor pool

Circular RNAs regulate neuron size and migration of midbrain dopamine neurons during development

Midbrain dopamine (mDA) neurons play an essential role in cognitive and motor behaviours and are linked to different brain disorders. However, the molecular mechanisms underlying their development, and in particular the role of non-coding RNAs (ncRNAs), remain incompletely understood. Here, we establish the transcriptomic landscape and alternative splicing patterns of circular RNAs (circRNAs) at key developmental timepoints in mouse mDA neurons in vivo using fluorescence-activated cell sorting followed by short- and long-read RNA sequencing. In situ hybridisation shows expression of several circRNAs during early mDA neuron development and post-transcriptional silencing unveils roles for different circRNAs in regulating mDA neuron morphology. Finally, in utero electroporation and time-lapse imaging implicate circRmst, a circRNA with widespread morphological effects, in the migration of developing mDA neurons in vivo. Together, these data for the first time suggest a functional role for circRNAs in developing mDA neurons and characterise poorly defined aspects of mDA neuron development.

Unraveling the role of miRNAs in the diagnosis, progression, and therapeutic intervention of Alzheimer's disease

Alzheimer's disease (AD) is a multifaceted, advancing neurodegenerative illness that is responsible for most cases of neurological impairment and dementia in the aged population. As the disease progresses, affected individuals may experience cognitive decline, linguistic problems, affective instability, and behavioral changes. The intricate nature of AD reflects the altered molecular mechanisms participating in the affected human brain. MicroRNAs (miRNAs, miR) are essential for the intricate control of gene expression in neurobiology. miRNAs exert their influence by modulating the transcriptome of brain cells, which typically exhibit substantial genetic activity, encompassing gene transcription and mRNA production. Presently, comprehensive studies are being conducted on AD to identify miRNA-based signatures that are indicative of the disease pathophysiology. These findings can contribute to the advancement of our understanding of the mechanisms underlying this disorder and can inform the development of therapeutic interventions based on miRNA and related RNA molecules. Therefore, this comprehensive review provides a detailed holistic analysis of the latest advances discussing the emerging role of miRNAs in the progression of AD and their possible application as potential biomarkers and targets for therapeutic interventions in future studies.

Unraveling the role of miRNAs in the diagnosis, progression, and therapeutic intervention of Parkinson's disease

Parkinson's disease (PD) is a debilitating neurological disorder characterized by the impairment of the motor system, resulting in symptoms such as resting tremor, cogwheel rigidity, bradykinesia, difficulty with gait, and postural instability. The occurrence of striatal dopamine insufficiency can be attributed to a notable decline in dopaminergic neurons inside the substantia nigra pars compacta. Additionally, the development of Lewy bodies serves as a pathological hallmark of PD. While current therapy approaches for PD aim to preserve dopaminergic neurons or replenish dopamine levels in the brain, it is important to acknowledge that achieving complete remission of the condition remains elusive. MicroRNAs (miRNAs, miR) are a class of small, non-coding ribonucleic acids involved in regulating gene expression at the post-transcriptional level. The miRNAs play a crucial part in the underlying pathogenic mechanisms of several neurodegenerative illnesses, including PD. The aim of this review is to explore the role of miRNAs in regulating genes associated with the onset and progression of PD, investigate the potential of miRNAs as a diagnostic tool, assess the effectiveness of targeting specific miRNAs as an alternative therapeutic strategy to impede disease advancement, and discuss the utilization of newly developed nanoparticles for delivering miRNAs as neurodegenerative therapies.

Astragalosides inhibit proliferation of fibroblast-like synoviocytes in experimental arthritis by modulating LncRNA S56464.1/miR-152-3p/Wnt1 signaling axis

AIM

Astragalus membranaceus (Fisch.) Bunge., the dried root of the plant A. membranaceus, is widely used in the treatment of rheumatoid arthritis (RA) in many Chinese herbal remedies. Astragalosides (AST) is the primary medicinal ingredient of A. membranaceus and has a therapeutic effect on RA, but the specific mechanism of this effect has yet to be elucidated.

METHODS

In this study, MTT and flow cytometry were used to determine the effects of AST on fibroblast-like synoviocyte (FLS) proliferation and cell cycle progression. Additionally, real-time quantitative polymerase chain reaction and Western blotting were used to determine the effects of AST on the LncRNA S56464.1/miR-152-3p/Wnt1 signaling axis and on critical genes that are essential to the Wnt pathway.

RESULTS

The data showed that after the administration of AST, FLS proliferation and LncRNA S56464.1, β-catenin, C-myc, Cyclin D1, and p-GSK-3β(Ser9)/GSK-3β expression were significantly reduced, and miR-152 and SFRP4 expression was notably increased.

CONCLUSION

These results suggest that AST can inhibit FLS proliferation by modulating the LncRNA S56464.1/miR-152-3p/Wnt1 signaling axis and that AST may be a potential therapeutic drug for RA.

WNT-β Catenin Signaling as a Potential Therapeutic Target for Neurodegenerative Diseases: Current Status and Future Perspective

Wnt/β-catenin (WβC) signaling pathway is an important signaling pathway for the maintenance of cellular homeostasis from the embryonic developmental stages to adulthood. The canonical pathway of WβC signaling is essential for neurogenesis, cell proliferation, and neurogenesis, whereas the noncanonical pathway (WNT/Ca²⁺ and WNT/PCP) is responsible for cell polarity, calcium maintenance, and cell migration. Abnormal regulation of WβC signaling is involved in the pathogenesis of several neurodegenerative diseases such as Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), amyotrophic lateral sclerosis (ALS), multiple sclerosis (MS), and spinal muscular atrophy (SMA). Hence, the alteration of WβC signaling is considered a potential therapeutic target for the treatment of neurodegenerative disease. In the present review, we have used the bibliographical information from PubMed, Google Scholar, and Scopus to address the current prospects of WβC signaling role in the abovementioned neurodegenerative diseases.

TGFβ3, dibutyryl cAMP and a notch inhibitor modulate phenotype late in stem cell-derived dopaminergic neuron maturation

The generation of midbrain dopaminergic neurons (mDAs) from pluripotent stem cells (hPSC) holds much promise for both disease modelling studies and as a cell therapy for Parkinson's disease (PD). Generally, dopaminergic neuron differentiation paradigms rely on inhibition of smad signalling for neural induction followed by hedgehog signalling and an elevation of β-catenin to drive dopaminergic differentiation. Post-patterning, differentiating dopaminergic neuron cultures are permitted time for maturation after which the success of these differentiation paradigms is usually defined by expression of tyrosine hydroxylase (TH), the rate limiting enzyme in the synthesis of dopamine. However, during maturation, culture media is often supplemented with additives to promote neuron survival and or promote cell differentiation. These additives include dibutyryl cyclic adenosine monophosphate (dbcAMP), transforming growth factor β3 (TGFβ3) and or the γ-secretase inhibitor (DAPT). While these factors are routinely added to cultures, their impact upon pluripotent stem cell-derived mDA phenotype is largely unclear. In this study, we differentiate pluripotent stem cells toward a dopaminergic phenotype and investigate how the omission of dbcAMP, TGFβ3 or DAPT, late in maturation, affects the regulation of multiple dopaminergic neuron phenotype markers. We now show that the removal of dbcAMP or TGFβ3 significantly and distinctly impacts multiple markers of the mDA phenotype (FOXA2, EN1, EN2, FOXA2, SOX6), while commonly increasing both MSX2 and NEUROD1 and reducing expression of both tyrosine hydroxylase and WNT5A. Removing DAPT significantly impacted MSX2, OTX2, EN1, and KCNJ6. In the absence of any stressful stimuli, we suggest that these culture additives should be viewed as mDA phenotype-modifying, rather than neuroprotective. We also suggest that their addition to cultures is likely to confound the interpretation of both transplantation and disease modelling studies.

Lmx1a-Dependent Activation of miR-204/211 Controls the Timing of Nurr1-Mediated Dopaminergic Differentiation

The development of midbrain dopaminergic (DA) neurons requires a fine temporal and spatial regulation of a very specific gene expression program. Here, we report that during mouse brain development, the microRNA (miR-) 204/211 is present at a high level in a subset of DA precursors expressing the transcription factor Lmx1a, an early determinant for DA-commitment, but not in more mature neurons expressing Th or Pitx3. By combining different in vitro model systems of DA differentiation, we show that the levels of Lmx1a influence the expression of miR-204/211. Using published transcriptomic data, we found a significant enrichment of miR-204/211 target genes in midbrain dopaminergic neurons where Lmx1a was selectively deleted at embryonic stages. We further demonstrated that miR-204/211 controls the timing of the DA differentiation by directly downregulating the expression of Nurr1, a late DA differentiation master gene. Thus, our data indicate the Lmx1a-miR-204/211-Nurr1 axis as a key component in the cascade of events that ultimately lead to mature midbrain dopaminergic neurons differentiation and point to miR-204/211 as the molecular switch regulating the timing of Nurr1 expression.

The potential of mechanistic information organised within the AOP framework to increase regulatory uptake of the developmental neurotoxicity (DNT) in vitro battery of assays

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Current in vivo DNT testing for regulatory purposes is not effective.

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In vitro assays anchored to key neurodevelopmental processes are available.

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Development of Adverse Outcome Pathways is required to increase mechanistic understanding of DNT effects.

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DNT Integrated Approaches to Testing and Assessment for various regulatory purposes should be developed.

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The OECD Guidance Document on use of in vitro DNT battery of assays is currently under development.

A major challenge in regulatory developmental neurotoxicity (DNT) assessment is lack of toxicological information for many compounds. Therefore, the Test Guidelines programme of the Organisation for Economic Cooperation and Development (OECD) took the initiative to coordinate an international collaboration between diverse stakeholders to consider integration of alternative approaches towards improving the current chemical DNT testing. During the past few years, a series of workshops was organized during which a consensus was reached that incorporation of a DNT testing battery that relies on in vitro assays anchored to key neurodevelopmental processes should be developed. These key developmental processes include neural progenitor cell proliferation, neuronal and oligodendrocyte differentiation, neural cell migration, neurite outgrowth, synaptogenesis and neuronal network formation, as well key events identified in the existing Adverse Outcome Pathways (AOPs). AOPs deliver mechanistic information on the causal links between molecular initiating event, intermediate key events and an adverse outcome of regulatory concern, providing the biological context to facilitate development of Integrated Approaches to Testing and Assessment (IATA) for various regulatory purposes. Developing IATA case studies, using mechanistic information derived from AOPs, is expected to increase scientific confidence for the use of in vitro methods within an IATA, thereby facilitating regulatory uptake. This manuscript summarizes the current state of international efforts to enhance DNT testing by using an in vitro battery of assays focusing on the role of AOPs in informing the development of IATA for different regulatory purposes, aiming to deliver an OECD guidance document on use of in vitro DNT battery of assays that include in vitro data interpretation.

Noncoding RNAs and Midbrain DA Neurons: Novel Molecular Mechanisms and Therapeutic Targets in Health and Disease

Midbrain dopamine neurons have crucial functions in motor and emotional control and their degeneration leads to several neurological dysfunctions such as Parkinson's disease, addiction, depression, schizophrenia, and others. Despite advances in the understanding of specific altered proteins and coding genes, little is known about cumulative changes in the transcriptional landscape of noncoding genes in midbrain dopamine neurons. Noncoding RNAs-specifically microRNAs and long noncoding RNAs-are emerging as crucial post-transcriptional regulators of gene expression in the brain. The identification of noncoding RNA networks underlying all stages of dopamine neuron development and plasticity is an essential step to deeply understand their physiological role and also their involvement in the etiology of dopaminergic diseases. Here, we provide an update about noncoding RNAs involved in dopaminergic development and metabolism, and the related evidence of these biomolecules for applications in potential treatments for dopaminergic neurodegeneration.

Injury to dopaminergic neurons development via the Lmx1a/Wnt1 autoregulatory loop induced by simazine

Simazine is a kind of persistent organic pollutant that is detected in both ground and water and has several routes of exposure. Here, we explored the mechanisms underlying simazine-related effects on dopaminergic neurons via development-related factors using mouse embryos and embryonic mesencephalic hybrid cell line (MN9D cells). We treated pregnant mice with 50 μg/kg bw, 200 μg/kg bw simazine from the 0.5 day to the 10.5 day of embryonic phase and MN9D cells with 600 μM simazine for 24 h to research the mechanism of dopaminergic neurons acute respond to simazine through preliminary experiments. Protein expressions of LIM homeobox transcription factor 1-alpha (Lmx1a) and LIM homeobox transcription factor 1-beta (Lmx1b) displayed a dose- and time-dependent increase after the exposure to simazine. In the 200 μg/kg bw of embryos and the 24h-600 μM of MN9D cells, protein levels of dopaminergic developmental factors were significantly upregulated, and dopaminergic function was significantly damaged for the abnormal expression of Dyt5b. We demonstrated simazine induced the injury to dopaminergic neurons via the Lmx1a/wingless-related integration site 1 (Wnt1) and Lmx1b pathways. In the transfection experiments, we knocked down Lmx1a and Lmx1b of cells to verify the potential target of simazine-induced injury to dopaminergic neurons, respectively. We detected the protein and mRNA levels of development-related genes of dopaminergic neurons and intracellular dopamine levels in different treatment groups. Based on our experiments' results, we demonstrated an acute response to 24 h-600 μM simazine treatment, the simazine-induced injury to dopaminergic neuronal which leads to abnormal dopamine levels and dopaminergic impairment is via the activation of the Lmx1a/Wnt1 autoregulatory loop. Lmx1a is a promising target in the search for the mechanisms underlying simazine-induced dopaminergic injury.

Parkinson's disease, aging and adult neurogenesis: Wnt/β-catenin signalling as the key to unlock the mystery of endogenous brain repair

A common hallmark of age-dependent neurodegenerative diseases is an impairment of adult neurogenesis. Wingless-type mouse mammary tumor virus integration site (Wnt)/β-catenin (WβC) signalling is a vital pathway for dopaminergic (DAergic) neurogenesis and an essential signalling system during embryonic development and aging, the most critical risk factor for Parkinson's disease (PD). To date, there is no known cause or cure for PD. Here we focus on the potential to reawaken the impaired neurogenic niches to rejuvenate and repair the aged PD brain. Specifically, we highlight WβC-signalling in the plasticity of the subventricular zone (SVZ), the largest germinal region in the mature brain innervated by nigrostriatal DAergic terminals, and the mesencephalic aqueduct-periventricular region (Aq-PVR) Wnt-sensitive niche, which is in proximity to the SNpc and harbors neural stem progenitor cells (NSCs) with DAergic potential. The hallmark of the WβC pathway is the cytosolic accumulation of β-catenin, which enters the nucleus and associates with T cell factor/lymphoid enhancer binding factor (TCF/LEF) transcription factors, leading to the transcription of Wnt target genes. Here, we underscore the dynamic interplay between DAergic innervation and astroglial-derived factors regulating WβC-dependent transcription of key genes orchestrating NSC proliferation, survival, migration and differentiation. Aging, inflammation and oxidative stress synergize with neurotoxin exposure in "turning off" the WβC neurogenic switch via down-regulation of the nuclear factor erythroid-2-related factor 2/Wnt-regulated signalosome, a key player in the maintenance of antioxidant self-defense mechanisms and NSC homeostasis. Harnessing WβC-signalling in the aged PD brain can thus restore neurogenesis, rejuvenate the microenvironment, and promote neurorescue and regeneration.

Classification of Midbrain Dopamine Neurons Using Single-Cell Gene Expression Profiling Approaches

Dysfunctional dopamine (DA) signaling has been associated with a broad spectrum of neuropsychiatric disorders, prompting investigations into how midbrain DA neuron heterogeneity may underpin this variety of behavioral symptoms. Emerging literature indeed points to functional heterogeneity even within anatomically defined DA clusters. Recognizing the need for a systematic classification scheme, several groups have used single-cell profiling to catalog DA neurons based on their gene expression profiles. We aim here not only to synthesize points of congruence but also to highlight key differences between the molecular classification schemes derived from these studies. In doing so, we hope to provide a common framework that will facilitate investigations into the functions of DA neuron subtypes in the healthy and diseased brain.

The Basic Helix-Loop-Helix Gene Nato3 Drives Expression of Dopaminergic Neuron Transcription Factors in Neural Progenitors

The floor plate of the developing midbrain gives rise to dopaminergic (DA) neurons, an important class of cells involved in Parkinson's disease (PD). Neural progenitors of the midbrain floor plate utilize key genes in transcriptional networks to drive dopamine neurogenesis. Identifying factors that promote dopaminergic neuron transcriptional networks can provide insight into strategies for therapies in PD. Using the chick embryo, we developed a quantitative PCR (qPCR) based method to assess the potential of a candidate factor to drive DA neuron gene expression, including the basic helix-loop-helix transcription factor Nato3 (Ferd3l). We then showed that overexpression of Nato3 in the developing chick mesencephalon produces a regionally dependent increase in genes associated with the DA neurogenesis, (such as Foxa2, Lmx1b and Shh) as well as DA neuron genes Nurr1 (an immature DA neuron marker) and mRNA expression of tyrosine hydroxylase (TH, a mature DA neuron marker). Interestingly, our data also showed that Nato3 is a potent regulator of Lmx1b by its broad induction of Lmx1b expression in neural progenitors of multiple regions of the CNS, including the midbrain and spinal cord. These data introduce a new, in vivo approach to identifying a gene that can drive DA transcriptional networks and provide the new insight that Nato3 can drive expression of key DA neuron genes, including Lmx1b, in neural progenitors.

MicroRNA-Based Separation of Cortico-Fugal Projection Neuron-Like Cells Derived From Embryonic Stem Cells

The purification of pluripotent stem cell-derived cortico-fugal projection neurons (PSC-CFuPNs) is useful for disease modeling and cell therapies related to the dysfunction of cortical motor neurons, such as amyotrophic lateral sclerosis (ALS) or stroke. However, no CFuPN-specific surface markers for the purification are known. Recently, microRNAs (miRNAs) have been reported as alternatives to surface markers. Here, we investigated this possibility by applying the miRNA switch, an mRNA technology, to enrich PSC-CFuPNs. An array study of miRNAs in mouse fetal brain tissue revealed that CFuPNs highly express miRNA-124-3p at E14.5 and E16.5. In response, we designed a miRNA switched that responds to miRNA-124-3p and applied it to mouse embryonic stem cell (ESC)-derived cortical neurons. Flow cytometry and quantitative polymerase chain reaction (qPCR) analyses showed the miRNA-124-3p switch enriched CFuPN-like cells from this population. Immunocytechemical analysis confirmed vGlut1/Emx1/Bcl11b triple positive CFuPN-like cells were increased from 6.5 to 42%. Thus, our miRNA-124-3p switch can uniquely enrich live CFuPN-like cells from mouse ESC-derived cortical neurons.

Nrf2-regulated miR-380-3p Blocks the Translation of Sp3 Protein and Its Mediation of Paraquat-Induced Toxicity in Mouse Neuroblastoma N2a Cells

Laboratorial and epidemiological research has established a relationship between paraquat (PQ) exposure and a risk for Parkinson's disease. Previously, we have investigated the effects of nuclear factor erythroid 2 related factor 2 (Nrf2) and microRNAs in PQ-induced neurotoxicity, addressing the function of miR-380-3p, a microRNA dysregulated by PQ, as well as Nrf2 deficiency. Nrf2 is known to mediate the expression of a variety of genes, including noncoding genes. By chromatin immunoprecipitation, we identified the relationship between Nrf2 and miR-380-3p in transcriptional regulation. qRT-PCR, Western blots, and dual-luciferase reporter gene assay showed that miR-380-3p blocked the translation of the transcription factor specificity protein-3 (Sp3) in the absence of degradation of Sp3 mRNA. Results based on cell counting analysis, annexin v-fluorescein isothiocyanate/propidium iodide double-staining assay, and propidium iodide staining showed that overexpression of miR-380-3p inhibited cell proliferation, increased the apoptotic rate, induced cell cycle arrest, and intensified the toxicity of PQ in mouse neuroblastoma (N2a [Neuro2a]) cells. Knockdown of Sp3 inhibited cell proliferation and eclipsed the alterations induced by miR-380-3p in cell proliferation. Two mediators of apoptosis and cell cycle identified in previous studies as Sp3-regulated, namely cyclin-dependent kinase inhibitor 1 (p21) and calmodulin (CaM), were dysregulated by PQ, but not Sp3 deficiency. In conclusion, Nrf2-regulated miR-380-3p inhibited cell proliferation and enhanced the PQ-induced toxicity in N2a cells potentially by blocking the translation Sp3 mRNA. We conclude that CaM and p21 were involved in PQ-induced toxicity.

Serotonin is elevated in risk-genotype carriers of TCF7L2 - rs7903146

The transcription factor 7-like 2 (TCF7L2) polymorphism rs7903146 is known to be tightly associated with an elevated risk for type 2 diabetes, whereas the molecular mechanisms remain elusive. We evaluated the metabolic profile of a total of 394 patients' serum samples with respect to their rs7903146 genotype using targeted metabolomics in a discovery (n = 154) and a validation (n = 240) study. We have identified serotonin as the top metabolite being increased in carriers of the risk allele. Serotonin was significantly associated with the rs7903146 genotype after full adjustment including type 2 diabetes and further top ranked metabolites. Given the role of peripheral serotonin in metabolic homeostasis and type 2 diabetes, this finding provides a first hint that the well-known impact of the TCF7L2 polymorphism on type 2 diabetes risk may involve a serotonin-dependent pathway.

Genome-wide brain DNA methylation analysis suggests epigenetic reprogramming in Parkinson disease

Objective

Given the known strong relationship of DNA methylation with environmental exposure, we investigated whether brain regions affected in Parkinson disease (PD) were differentially methylated between PD cases and controls.

Methods

DNA chip arrays were used to perform a genome-wide screen of DNA methylation on the dorsal motor nucleus of the vagus (DMV), substantia nigra (SN), and cingulate gyrus (CG) of pathologically confirmed PD cases and controls selected using the criteria of Beecham et al. Analysis examined differentially methylated regions (DMRs) between cases and controls for each brain area. RNA sequencing and pathway analysis were also performed for each brain area.

Results

Thirty-eight PD cases and 41 controls were included in the analysis. Methylation studies revealed 234 significant DMR in the DMV, 44 in the SN, and 141 in the CG between cases and controls (Sidak p < 0.05). Pathway analysis of these genes showed significant enrichment for the Wnt signaling pathway (FDR < 0.01).

Conclusions

Our data suggest that significant DNA methylation changes exist between cases and controls in PD, especially in the DMV, one of the areas affected earliest in PD. The etiology of these methylation changes is not yet known, but the predominance of methylation changes occurring in the DMV supports the hypothesis that vagus nerve function, perhaps involving the gastrointestinal system, is important in PD pathogenesis. These data also give independent support that genes involved in Wnt signaling are a likely factor in the neurodegenerative processes of PD.

MiR-135a-5p Is Critical for Exercise-Induced Adult Neurogenesis

Physical exercise stimulates adult hippocampal neurogenesis and is considered a relevant strategy for preventing age-related cognitive decline in humans. The underlying mechanisms remains controversial. Here, we show that exercise increases proliferation of neural precursor cells (NPCs) of the mouse dentate gyrus (DG) via downregulation of microRNA 135a-5p (miR-135a). MiR-135a inhibition stimulates NPC proliferation leading to increased neurogenesis, but not astrogliogenesis, in DG of resting mice, and intriguingly it re-activates NPC proliferation in aged mice. We identify 17 proteins (11 putative targets) modulated by miR-135 in NPCs. Of note, inositol 1,4,5-trisphosphate (IP3) receptor 1 and inositol polyphosphate-4-phosphatase type I are among the modulated proteins, suggesting that IP3 signaling may act downstream miR-135. miR-135 is the first noncoding RNA essential modulator of the brain's response to physical exercise. Prospectively, the miR-135-IP3 axis might represent a novel target of therapeutic intervention to prevent pathological brain aging.

Pharmacological Transdifferentiation of Human Nasal Olfactory Stem Cells into Dopaminergic Neurons

The discovery of novel drugs for neurodegenerative diseases has been a real challenge over the last decades. The development of patient- and/or disease-specific in vitro models represents a powerful strategy for the development and validation of lead candidates in preclinical settings. The implementation of a reliable platform modeling dopaminergic neurons will be an asset in the study of dopamine-associated pathologies such as Parkinson's disease. Disease models based on cell reprogramming strategies, using either human-induced pluripotent stem cells or transcription factor-mediated transdifferentiation, are among the most investigated strategies. However, multipotent adult stem cells remain of high interest to devise direct conversion protocols and establish in vitro models that could bypass certain limitations associated with reprogramming strategies. Here, we report the development of a six-step chemically defined protocol that drives the transdifferentiation of human nasal olfactory stem cells into dopaminergic neurons. Morphological changes were progressively accompanied by modifications matching transcript and protein dopaminergic signatures such as LIM homeobox transcription factor 1 alpha (LMX1A), LMX1B, and tyrosine hydroxylase (TH) expression, within 42 days of differentiation. Phenotypic changes were confirmed by the production of dopamine from differentiated neurons. This new strategy paves the way to develop more disease-relevant models by establishing reprogramming-free patient-specific dopaminergic cell models for drug screening and/or target validation for neurodegenerative diseases.

Antagonizing Increased miR-135a Levels at the Chronic Stage of Experimental TLE Reduces Spontaneous Recurrent Seizures

Mesial temporal lobe epilepsy (mTLE) is a chronic neurological disease characterized by recurrent seizures. The antiepileptic drugs currently available to treat mTLE are ineffective in one-third of patients and lack disease-modifying effects. miRNAs, a class of small noncoding RNAs which control gene expression at the post-transcriptional level, play a key role in the pathogenesis of mTLE and other epilepsies. Although manipulation of miRNAs at acute stages has been reported to reduce subsequent spontaneous seizures, it is uncertain whether targeting miRNAs at chronic stages of mTLE can also reduce seizures. Furthermore, the functional role and downstream targets of most epilepsy-associated miRNAs remain poorly understood. Here, we show that miR-135a is selectively upregulated within neurons in epileptic brain and report that targeting miR-135a in vivo using antagomirs after onset of spontaneous recurrent seizures can reduce seizure activity at the chronic stage of experimental mTLE in male mice. Further, by using an unbiased approach combining immunoprecipitation and RNA sequencing, we identify several novel neuronal targets of miR-135a, including Mef2a Mef2 proteins are key regulators of excitatory synapse density. Mef2a and miR-135a show reciprocal expression regulation in human (of both sexes) and experimental TLE, and miR-135a regulates dendritic spine number and type through Mef2. Together, our data show that miR-135a is target for reducing seizure activity in chronic epilepsy, and that deregulation of miR-135a in epilepsy may alter Mef2a expression and thereby affect synaptic function and plasticity.SIGNIFICANCE STATEMENT miRNAs are post-transcriptional regulators of gene expression with roles in the pathogenesis of epilepsy. However, the precise mechanism of action and therapeutic potential of most epilepsy-associated miRNAs remain poorly understood. Our study reveals dramatic upregulation of the key neuronal miRNA miR-135a in both experimental and human mesial temporal lobe epilepsy. Silencing miR-135a in experimental temporal lobe epilepsy reduces seizure activity at the spontaneous recurrent seizure stage. These data support the exciting possibility that miRNAs can be targeted to combat seizures after spontaneous seizure activity has been established. Further, by using unbiased approaches novel neuronal targets of miR-135a, including members of the Mef2 protein family, are identified that begin to explain how deregulation of miR-135a may contribute to epilepsy.

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.

Crosstalk of Intercellular Signaling Pathways in the Generation of Midbrain Dopaminergic Neurons In Vivo and from Stem Cells

Dopamine-synthesizing neurons located in the mammalian ventral midbrain are at the center stage of biomedical research due to their involvement in severe human neuropsychiatric and neurodegenerative disorders, most prominently Parkinson’s Disease (PD). The induction of midbrain dopaminergic (mDA) neurons depends on two important signaling centers of the mammalian embryo: the ventral midline or floor plate (FP) of the neural tube, and the isthmic organizer (IsO) at the mid-/hindbrain boundary (MHB). Cells located within and close to the FP secrete sonic hedgehog (SHH), and members of the wingless-type MMTV integration site family (WNT1/5A), as well as bone morphogenetic protein (BMP) family. The IsO cells secrete WNT1 and the fibroblast growth factor 8 (FGF8). Accordingly, the FGF8, SHH, WNT, and BMP signaling pathways play crucial roles during the development of the mDA neurons in the mammalian embryo. Moreover, these morphogens are essential for the generation of stem cell-derived mDA neurons, which are critical for the modeling, drug screening, and cell replacement therapy of PD. This review summarizes our current knowledge about the functions and crosstalk of these signaling pathways in mammalian mDA neuron development in vivo and their applications in stem cell-based paradigms for the efficient derivation of these neurons in vitro.

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

Mapping projections of molecularly defined dopamine neuron subtypes using intersectional genetic approaches

Midbrain dopamine (DA) neurons have diverse functions that can in part be explained by their heterogeneity. Although molecularly distinct subtypes of DA neurons have been identified by single-cell gene expression profiling, fundamental features such as their projection patterns have not been elucidated. Progress in this regard has been hindered by the lack of genetic tools for studying DA neuron subtypes. Here we develop intersectional genetic labeling strategies, based on combinatorial gene expression, to map the projections of molecularly defined DA neuron subtypes. We reveal distinct genetically defined dopaminergic pathways arising from the substantia nigra pars compacta and from the ventral tegmental area that innervate specific regions of the caudate putamen, nucleus accumbens and amygdala. Together, the genetic toolbox and DA neuron subtype projections presented here constitute a resource that will accelerate the investigation of this clinically significant neurotransmitter system.

Therapeutic Antidepressant Potential of NONHSAG045500 in Regulating Serotonin Transporter in Major Depressive Disorder

Background

Major depressive disorder (MDD) is a chronic, life-threatening, highly disabling disease. Standardized treatment with fewer adverse effects, quick onset, and long-term maintenance of the effects of brief treatment for MDD is always being pursued. Long non-coding RNAs (lncRNAs) are highly expressed in the central nervous system and are involved in the occurrence and development of neurodegenerative and psychiatric diseases. This study aimed to investigate whether the overexpression and interference of 3 differentially down-regulated lncRNAs (NONHSAT142707, NONHSAG045500, and ENST00000517573) in MDD can affect the expression of central neurotransmitter serotonin (5-hydroxytryptamine) transporter (SERT) in vitro.

Material/Methods

First, we synthesized and validated the effect of 3 lncRNA plasmids and small interfering RNAs (siRNAs); next, we transfected the plasmids and siRNAs that caused significant overexpression or interference in SK-N-SH cells, and tested the expression of SERT by qRT-PCR.

Results

The results showed that 3 lncRNA plasmids and siRNAs2 caused overexpression and interference, respectively. Only the overexpression of NONHSAG045500 could significantly inhibit the expression of SERT; interference with NONHSAG045500 could significantly strengthen the expression of SERT.

Conclusions

This study indicated that the expression of SERT could be regulated by up-regulating or down-regulating NONHSAG045500 expression and suggested that NONHSAG045500 could potentially be established as a new therapeutic target of MDD. Future work may be needed to definitively determine the correlation between NONHSAG045500 and SERT in vivo.

MicroRNA alterations in iPSC-derived dopaminergic neurons from Parkinson disease patients

MicroRNA (miRNA) misregulation in peripheral blood has been linked to Parkinson disease (PD) but its role in the disease progression remains elusive. We performed an explorative genome-wide study of miRNA expression levels in dopaminergic neurons (DAn) from PD patients generated by somatic cell reprogramming and induced pluripotent stem cells differentiation. We quantified expression levels of 377 miRNAs in DAn from 3 sporadic PD patients (sPD), 3 leucine-rich repeat kinase 2-associated PD patients (L2PD) (total 6 PD), and 4 healthy controls. We identified differential expression of 10 miRNA of which 5 were upregulated in PD (miR-9-5p, miR-135a-5p, miR-135b-5p, miR-449a, and miR-449b-5p) and 5 downregulated (miR-141-3p, miR-199a-5p, miR-299-5p, miR-518e-3p, and miR-519a-3p). Changes were similar in sPD and L2PD. Integrative analysis revealed significant correlations between miRNA/mRNA expression. Moreover, upregulation of miR-9-5p and miR-135b-5p was associated with downregulation of transcription factors related to the DNA hypermethylation of enhancer elements in PD DAn (FOXA1 and NR3C1). In summary, miRNA changes are associated with monogenic L2PD and sPD and co-occur with epigenetic changes in DAn from PD patients.

miR-34b/c Regulates Wnt1 and Enhances Mesencephalic Dopaminergic Neuron Differentiation

The differentiation of dopaminergic neurons requires concerted action of morphogens and transcription factors acting in a precise and well-defined time window. Very little is known about the potential role of microRNA in these events. By performing a microRNA-mRNA paired microarray screening, we identified miR-34b/c among the most upregulated microRNAs during dopaminergic differentiation. Interestingly, miR-34b/c modulates Wnt1 expression, promotes cell cycle exit, and induces dopaminergic differentiation. When combined with transcription factors ASCL1 and NURR1, miR-34b/c doubled the yield of transdifferentiated fibroblasts into dopaminergic neurons. Induced dopaminergic (iDA) cells synthesize dopamine and show spontaneous electrical activity, reversibly blocked by tetrodotoxin, consistent with the electrophysiological properties featured by brain dopaminergic neurons. Our findings point to a role for miR-34b/c in neuronal commitment and highlight the potential of exploiting its synergy with key transcription factors in enhancing in vitro generation of dopaminergic neurons.

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miR-34b/c is enriched in Pitx3-GFP⁺ mDA neurons

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miR-34b/c targets Wnt1-3′ UTR

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miR-34b/c is expressed during dopaminergic differentiation of mESCs

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miR-34b/c enhances fibroblast transdifferentiation into functional iDA neurons

Understanding the Role of lncRNAs in Nervous System Development

The diversity of lncRNAs has expanded within mammals in tandem with the evolution of increased brain complexity, suggesting that lncRNAs play an integral role in this process. In this chapter, we will highlight the identification and characterization of lncRNAs in nervous system development. We discuss the potential role of lncRNAs in nervous system and brain evolution, along with efforts to create comprehensive catalogues that analyze spatial and temporal changes in lncRNA expression during nervous system development. Additionally, we focus on recent endeavors that attempt to assign function to lncRNAs during nervous system development. We highlight discrepancies that have been observed between in vitro and in vivo studies of lncRNA function and the challenges facing researchers in conducting mechanistic analyses of lncRNAs in the developing nervous system. Altogether, this chapter highlights the emerging role of lncRNAs in the developing brain and sheds light on novel, RNA-mediated mechanisms by which nervous system development is controlled.

MicroRNAs in neural development: from master regulators to fine-tuners

The proper formation and function of neuronal networks is required for cognition and behavior. Indeed, pathophysiological states that disrupt neuronal networks can lead to neurodevelopmental disorders such as autism, schizophrenia or intellectual disability. It is well-established that transcriptional programs play major roles in neural circuit development. However, in recent years, post-transcriptional control of gene expression has emerged as an additional, and probably equally important, regulatory layer. In particular, it has been shown that microRNAs (miRNAs), an abundant class of small regulatory RNAs, can regulate neuronal circuit development, maturation and function by controlling, for example, local mRNA translation. It is also becoming clear that miRNAs are frequently dysregulated in neurodevelopmental disorders, suggesting a role for miRNAs in the etiology and/or maintenance of neurological disease states. Here, we provide an overview of the most prominent regulatory miRNAs that control neural development, highlighting how they act as ‘master regulators’ or ‘fine-tuners’ of gene expression, depending on context, to influence processes such as cell fate determination, cell migration, neuronal polarization and synapse formation.

A novel floor plate boundary defined by adjacent En1 and Dbx1 microdomains distinguishes midbrain dopamine and hypothalamic neurons

The mesodiencephalic floor plate (mdFP) is the source of diverse neuron types. Yet, how this structure is compartmentalized has not been clearly elucidated. Here, we identify a novel boundary subdividing the mdFP into two microdomains, defined by engrailed 1 (En1) and developing brain homeobox 1 (Dbx1). Utilizing simultaneous dual and intersectional fate mapping, we demonstrate that this boundary is precisely formed with minimal overlap between En1 and Dbx1 microdomains, unlike many other boundaries. We show that the En1 microdomain gives rise to dopaminergic (DA) neurons, whereas the Dbx1 microdomain gives rise to subthalamic (STN), premammillary (PM) and posterior hypothalamic (PH) populations. To determine whether En1 is sufficient to induce DA neuron production beyond its normal limit, we generated a mouse strain that expresses En1 in the Dbx1 microdomain. In mutants, we observed ectopic production of DA neurons derived from the Dbx1 microdomain, at the expense of STN and PM populations. Our findings provide new insights into subdivisions in the mdFP, and will impact current strategies for the conversion of stem cells into DA neurons.

The effects of short-term high-fat feeding on exercise capacity: multi-tissue transcriptome changes by RNA sequencing analysis

BACKGROUND

The effects of short-term high fat diets on physiology are elusive and the molecular changes following fat overconsumption remain largely unknown. In this study, we aimed to evaluate exercise capacity in mice fed with a high fat diet (HFD) for 3 days and investigate the molecular mechanisms in the early response to high-fat feeding.

METHODS

Exercise capacity was assessed by weight-loaded swimming test in mice fed a control diet (10 kcal% fat) or a HFD (60 kcal% fat) for 3 days. Global gene expression of ten important tissues (brain, heart, liver, spleen, lung, kidney, stomach, duodenum, skeletal muscle and blood) was analyzed using RNA Sequencing.

RESULTS

A HFD for just 3 days can induce 71% decrease of exercise performance prior to substantial weight gain (P <0.01). Principle component analysis revealed that differential gene expression patterns existed in the ten tissues. Out of which, the brain, spleen and lung were demonstrated to have more pronounced transcriptional changes than other tissues. Biological process analysis for differentially expressed genes in the brain, spleen and lung showed that dysregulation of peripheral and central immune response had been implicated in the early stage of HFD exposure. Neurotransmission related genes and circulatory system process related genes were significantly down-regulated in the brain and lung, respectively.

CONCLUSIONS

Our findings provide new insights for the deleterious effects of high-fat feeding, especially revealing that the lung maybe as a new important target attacked by short-term high-fat feeding.

Neurodevelopmental Perspectives on Wnt Signaling in Psychiatry

Mounting evidence indicates that Wnt signaling is relevant to pathophysiology of diverse mental illnesses including schizophrenia, bipolar disorder, and autism spectrum disorder. In the 35 years since Wnt ligands were first described, animal studies have richly explored how downstream Wnt signaling pathways affect an array of neurodevelopmental processes and how their disruption can lead to both neurological and behavioral phenotypes. Recently, human induced pluripotent stem cell (hiPSC) models have begun to contribute to this literature while pushing it in increasingly translational directions. Simultaneously, large-scale human genomic studies are providing evidence that sequence variation in Wnt signal pathway genes contributes to pathogenesis in several psychiatric disorders. This article reviews neurodevelopmental and postneurodevelopmental functions of Wnt signaling, highlighting mechanisms, whereby its disruption might contribute to psychiatric illness, and then reviews the most reliable recent genetic evidence supporting that mutations in Wnt pathway genes contribute to psychiatric illness. We are proponents of the notion that studies in animal and hiPSC models informed by the human genetic data combined with the deep knowledge base and tool kits generated over the last several decades of basic neurodevelopmental research will yield near-term tangible advances in neuropsychiatry.

Noncoding RNA Regulation of Dopamine Signaling in Diseases of the Central Nervous System

Dopaminergic neurotransmission mediates a majority of the vital central nervous system functions. Disruption of these synaptic events provokes a multitude of neurological pathologies, including Parkinson's, schizophrenia, depression, and addiction. Growing evidence supports a key role for noncoding RNA (ncRNA) regulation in the synapse. This review will discuss the role of both short and long ncRNAs in dopamine signaling, including bioinformatic examination of the pathways they target. Specifically, we focus on the contribution of ncRNAs to dopaminergic dysfunction in neurodegenerative as well as psychiatric disease.

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.

Epigenetic regulation contributes to urocortin-enhanced midbrain dopaminergic neuron differentiation

The production of midbrain dopaminergic (mDA) neurons requires precise extrinsic inductive signals and intrinsic transcriptional cascade at a specific time point in development. Urocortin (UCN) is a peptide of the corticotropin-releasing hormone family that mediates various responses to stress. UCN was first cloned from adult rat midbrain. However, the contribution of UCN to the development of mDA neurons is poorly understood. Here, we show that UCN is endogenously expressed in the developing ventral midbrain (VM) and its receptors are exhibited in Nurr1(+) postmitotic mDA precursors and TH(+) neurons, suggesting possible roles in regulating their terminal differentiation. UCN treatment increased DA cell numbers in rat VM precursor cultures by promoting the conversion of Nurr1(+) precursors into DA neurons. Furthermore, neutralization of secreted UCN with anti-UCN antibody resulted in a reduction in the number of DA neurons. UCN induced an abundance of acetylated histone H3 and enhanced late DA regulator Nurr1, Foxa2, and Pitx3 expressions. Using pharmacological and RNA interference approaches, we further demonstrated that histone deacetylase (HDAC) inhibition and late transcriptional factors upregulation contribute to UCN-mediated DA neuron differentiation. Chromatin immunoprecipitation analyses revealed that UCN promoted histone acetylation of chromatin surrounding the TH promoter by directly inhibiting HDAC and releasing of methyl CpG binding protein 2-CoREST-HDAC1 repressor complex from the promoter, ultimately leading to an increase in Nurr1/coactivators-mediated transcription of TH gene. Moreover, UCN treatment in vivo also resulted in increased DA neuron differentiation. These findings suggest that UCN might contribute to regulate late mDA neuron differentiation during VM development.

Post-transcriptional regulation of gene expression in neural stem cells

Expression of each gene can be controlled at several steps during the flow of genetic information from DNA to protein. Tight regulation of gene expression is especially important for stem cells because of their greater ripple effects, compared with terminally differentiated cells. Dysregulation of gene expression arising in stem cells can be perpetuated within the stem cell pool via self-renewal throughout life. In addition, transcript profiles within stem cells can determine the selective advantage or disadvantage of each cell, leading to changes in cell fate, such as a tendency for proliferation, death, and differentiation. The identification of neural stem/progenitor cells (NSPCs) and greater understanding of their cellular physiology have raised the possibility of using NSPCs to replace damaged or injured neurons. However, an accurate grasp of gene expression control must take precedence in order to use NSPCs in therapies for neurological diseases. Recently, accumulating evidence has demonstrated the importance of post-transcriptional regulation in NSPC fate decisions. In this review, we will summarize and discuss the recent findings on key mRNA modulators and their vital roles in NSPC homeostasis. Copyright © 2016 John Wiley & Sons, Ltd.

MicroRNAs: Not "Fine-Tuners" but Key Regulators of Neuronal Development and Function

MicroRNAs (miRNAs) are a class of short non-coding RNAs that operate as prominent post-transcriptional regulators of eukaryotic gene expression. miRNAs are abundantly expressed in the brain of most animals and exert diverse roles. The anatomical and functional complexity of the brain requires the precise coordination of multilayered gene regulatory networks. The flexibility, speed, and reversibility of miRNA function provide precise temporal and spatial gene regulatory capabilities that are crucial for the correct functioning of the brain. Studies have shown that the underlying molecular mechanisms controlled by miRNAs in the nervous systems of invertebrate and vertebrate models are remarkably conserved in humans. We endeavor to provide insight into the roles of miRNAs in the nervous systems of these model organisms and discuss how such information may be used to inform regarding diseases of the human brain.

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.

Otx2 Requires Lmx1b to Control the Development of Mesodiencephalic Dopaminergic Neurons

Studying the development of mesodiencephalic dopaminergic (mdDA) neurons provides an important basis for better understanding dopamine-associated brain functions and disorders and is critical for establishing cell replacement therapy for Parkinson’s disease. The transcription factors Otx2 and Lmx1b play a key role in the development of mdDA neurons. However, little is known about the genes downstream of Otx2 and Lmx1b in the pathways controlling the formation of mdDA neurons in vivo. Here we report on our investigation of Lmx1b as downstream target of Otx2 in the formation of mdDA neurons. Mouse mutants expressing Otx2 under the control of the En1 promoter (En1^+/Otx2) showed increased Otx2 expression in the mid-hindbrain region, resulting in upregulation of Lmx1b and expansion of mdDA neurons there. In contrast, Lmx1b^-/- mice showed decreased expression of Otx2 and impairments in several aspects of mdDA neuronal formation. To study the functional interaction between Otx2 and Lmx1b, we generated compound mutants in which Otx2 expression was restored in mice lacking Lmx1b (En1^+/Otx2;Lmx1b^-/-). In these animals Otx2 was not sufficient to rescue any of the aberrations in the formation of mdDA neurons caused by the loss of Lmx1b, but rescued the loss of ocular motor neurons. Gene expression studies in Lmx1b^-/- embryos indicated that in these mutants Wnt1, En1 and Fgf8 expression are induced but subsequently lost in the mdDA precursor domain and the mid-hindbrain organizer in a specific, spatio-temporal manner. In summary, we demonstrate that Otx2 critically depends on Lmx1b for the formation of mdDA neurons, but not for the generation of ocular motor neurons. Moreover, our data suggest that Lmx1b precisely maintains the expression pattern of Wnt1, Fgf8 and En1, which are essential for mid-hindbrain organizer function and the formation of mdDA neurons.

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.

Excessive Wnt/beta-catenin signaling promotes midbrain floor plate neurogenesis, but results in vacillating dopamine progenitors

The floor plate (FP), a ventral midline structure of the developing neural tube, has differential neurogenic capabilities along the anterior-posterior axis. The midbrain FP, unlike the hindbrain and spinal cord floor plate, is highly neurogenic and produces midbrain dopaminergic (mDA) neurons. Canonical Wnt/beta-catenin signaling, at least in part, is thought to account for the difference in neurogenic capability. Removal of beta-catenin results in mDA progenitor specification defects as well as a profound reduction of neurogenesis. To examine the effects of excessive Wnt/beta-catenin signaling on mDA specification and neurogenesis, we have analyzed a model wherein beta-catenin is conditionally stabilized in the Shh+domain. Here, we show that the Foxa2+/Lmx1a+ domain is extended rostrally in mutant embryos, suggesting that canonical Wnt/beta-catenin signaling can drive FP expansion along the rostrocaudal axis. Although excess canonical Wnt/beta-catenin signaling generally promotes neurogenesis at midbrain levels, less tyrosine hydroxylase (Th)+, mDA neurons are generated, particularly impacting the Substantia Nigra pars compacta. This is likely because of improper progenitor specification. Excess canonical Wnt/beta-catenin signaling causes downregulation of net Lmx1b, Shh and Foxa2 levels in mDA progenitors. Moreover, these progenitors assume a mixed identity to that of Lmx1a+/Lmx1b+/Nkx6-1+/Neurog1+ progenitors. We also show by lineage tracing analysis that normally, Neurog1+ progenitors predominantly give rise to Pou4f1+ neurons, but not Th+ neurons. Accordingly, in the mutant embryos, Neurog1+ progenitors at the midline generate ectopic Pou4f1+ neurons at the expense of Th+ mDA neurons. Our study suggests that an optimal dose of Wnt/beta-catenin signaling is critical for proper establishment of the mDA progenitor character. Our findings will impact embryonic stem cell protocols that utilize Wnt pathway reagents to derive mDA neuron models and therapeutics for Parkinson's disease.

microRNA regulation of Wnt signaling pathways in development and disease

Wnt signaling pathways and microRNAs (miRNAs) are critical regulators of development. Aberrant Wnt signaling pathways and miRNA levels lead to developmental defects and diverse human pathologies including but not limited to cancer. Wnt signaling pathways regulate a plethora of cellular processes during embryonic development and maintain homeostasis of adult tissues. A majority of Wnt signaling components are regulated by miRNAs which are small noncoding RNAs that are expressed in both animals and plants. In animal cells, miRNAs fine tune gene expression by pairing primarily to the 3'untranslated region of protein coding mRNAs to repress target mRNA translation and/or induce target degradation. miRNA-mediated regulation of signaling transduction pathways is important in modulating dose-sensitive response of cells to signaling molecules. This review discusses components of the Wnt signaling pathways that are regulated by miRNAs in the context of development and diseases. A fundamental understanding of miRNA functions in Wnt signaling transduction pathways may yield new insight into crosstalks of regulatory mechanisms essential for development and disease pathophysiology leading to novel therapeutics.

miR-135 family members mediate podocyte injury through the activation of Wnt/β-catenin signaling

The upregulation of Wnt/β-catenin signaling occurs in virtually all types of kidney disease and is associated with podocyte injury. However, the precise mechanisms involved in the development of kidney disease remain to be elucidated. MicroRNAs (miRNAs or miRs) are a class of short non-coding RNAs and they have been shown to be regulators of gene expression, mainly by binding to the untranslated region (UTR) of mRNAs. The aim of the present study was to determine the role of the 2 members of the miR-135 family (miR-135a and miR-135b) in podocyte injury and to elucidate the mechanisms responsible for the damage to podocytes. The results revealed that miR-135a and miR-135b were upregulated in models of podocyte injury and in glomeruli isolated from patients with focal segmental glomerulosclerosis (FSGS). The ectopic expression of miR-135a and miR-135b led to severe podocyte injury and the disorder of the podocyte cytoskeleton. Our findings demonstrated that miR-135a and miR-135b activated Wnt/β-catenin signaling and induced the nuclear translocation of β-catenin. Using luciferase reporter assays, reverse transcription-quantitative polymerase chain reaction (RT-qPCR) and western blot analysis, glycogen synthase kinase 3β (GSK3β) was identified as a target gene of miR-135a and miR-135b. To the best of our knowledge, this is the first study to demonstrate that members of the miR-135 family (specifically miR-135a and miR-135b) regulate the expression of GSK3β, thus playing a role in the development of podocyte injury and the disorder of the podocyte cytoskeleton. This is an important finding as it may contribute to the development of novel therapeutics for podocyte injury-associated glomerulopathies.

Genetic findings in obsessive-compulsive disorder connect to brain-derived neutrophic factor and mammalian target of rapamycin pathways: implications for drug development

Traditional pharmacological approaches to the treatment of obsessive-compulsive disorder (OCD) are based on affecting serotonergic and dopaminergic transmission in the central nervous system. However, genetic epidemiology findings are pointing to glutamate pathways and developmental genes as etiological in OCD. A review of recent genetic findings in OCD is conducted, and bioinformatics approaches are used to locate pathways relevant to neuroprotection. The OCD susceptibility genes DLGAP1, RYR3, PBX1-MEIS2, LMX1A and candidate genes BDNF and GRIN2B are components of the neuronal growth, differentiation and neurogenesis pathways BDNF-mTOR. These pathways are emerging as a promising area of research for the development of neuroprotective pharmaceuticals. Emergent genetic epidemiologic data on OCD and repetitive behaviors may support new approaches for pharmacological discovery. Neuroprotective approaches that take into consideration glutamate-mediated BDNF-mTOR pathways are suggested by OCD susceptibility genes.

Making a mes: A transcription factor-microRNA pair governs the size of the midbrain and the dopaminergic progenitor pool

Canonical Wnt signaling is critical for midbrain dopaminergic progenitor specification, proliferation, and neurogenesis. Yet mechanisms that control Wnt signaling remain to be fully elucidated. Wnt1 is a key ligand in the embryonic midbrain, and directs proliferation, survival, specification and neurogenesis. In a recent study, we reveal that the transcription factor Lmx1b promotes Wnt1/Wnt signaling, and dopaminergic progenitor expansion, consistent with earlier studies. Additionally, Lmx1b drives expression of a non-coding RNA called Rmst, which harbors miR135a2 in its last intron. miR135a2 in turn targets Lmx1b as well as several Wnt pathway targets. Conditional overexpression of miR135a2 in the midbrain, particularly during an early time, results in a decreased dopaminergic progenitor pool, and less dopaminergic neurons, consistent with decreased Wnt signaling. We propose a model in which Lmx1b and miR135a2 influence levels of Wnt1 and Wnt signaling, and expansion of the dopaminergic progenitor pool. Further loss of function experiments and biochemical validation of targets will be critical to verify this model. Wnt agonists have recently been utilized for programming stem cells toward a dopaminergic fate in vitro, highlighting the importance of agents that modulate the Wnt pathway.

Genetic control of midbrain dopaminergic neuron development

Midbrain dopaminergic neurons are involved in regulating motor control, reward behavior, and cognition. Degeneration or dysfunction of midbrain dopaminergic neurons is implicated in several neuropsychiatric disorders such as Parkinson's disease, substance use disorders, depression, and schizophrenia. Understanding the developmental processes that generate midbrain dopaminergic neurons will facilitate the generation of dopaminergic neurons from stem cells for cell replacement therapies to substitute degenerating cells in Parkinson's disease patients and will forward our understanding on how functional diversity of dopaminergic neurons in the adult brain is established. Midbrain dopaminergic neurons develop in a multistep process. Following the induction of the ventral midbrain, a distinct dopaminergic progenitor domain is specified and dopaminergic progenitors undergo proliferation, neurogenesis, and differentiation. Subsequently, midbrain dopaminergic neurons acquire a mature dopaminergic phenotype, migrate to their final position and establish projections and connections to their forebrain targets. This review will discuss insights gained on the signaling network of secreted molecules, cell surface receptors, and transcription factors that regulate specification and differentiation of midbrain dopaminergic progenitors and neurons, from the induction of the ventral midbrain to the migration of dopaminergic neurons. For further resources related to this article, please visit the WIREs website.

CONFLICT OF INTEREST

The authors have declared no conflicts of interest for this article.

The role of microRNAs in human neural stem cells, neuronal differentiation and subtype specification

The impressive neuronal diversity found within the nervous system emerges from a limited pool of neural progenitor cells that proceed through different gene expression programs to acquire distinct cell fates. Here, we review recent evidence indicating that microRNAs (miRNAs) are critically involved in conferring neural cell identities during neural induction, neuronal differentiation and subtype specification. Several studies have shown that miRNAs act in concert with other gene regulatory factors and genetic switches to regulate the spatial and temporal expression profiles of important cell fate determinants. So far, most studies addressing the role of miRNAs during neurogenesis were conducted using animal models. With the advent of human pluripotent stem cells and the possibility to differentiate these into neural stem cells, we now have the opportunity to study miRNAs in a human context. More insight into the impact of miRNA-based regulation during neural fate choice could in the end be exploited to develop new strategies for the generation of distinct human neuronal cell types.

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.