Tlx1/3 and Ptf1a control the expression of distinct sets of transmitter and peptide receptor genes in the developing dorsal spinal cord. J Neurosci. 2012;32:8509-8520. [PMID: 22723691 DOI: 10.1523/JNEUROSCI.6301-11.2012]

RANBP17 Overexpression Restores Nucleocytoplasmic Transport and Ameliorates Neurodevelopment in Induced DYT1 Dystonia Motor Neurons

DYT1 dystonia is a debilitating neurological movement disorder, and it represents the most frequent and severe form of hereditary primary dystonia. There is currently no cure for this disease due to its unclear pathogenesis. In our previous study utilizing patient-specific motor neurons (MNs), we identified distinct cellular deficits associated with the disease, including a deformed nucleus, disrupted neurodevelopment, and compromised nucleocytoplasmic transport (NCT) functions. However, the precise molecular mechanisms underlying these cellular impairments have remained elusive. In this study, we revealed the genome-wide changes in gene expression in DYT1 MNs through transcriptomic analysis. We found that those dysregulated genes are intricately involved in neurodevelopment and various biological processes. Interestingly, we identified that the expression level of RANBP17, a RAN-binding protein crucial for NCT regulation, exhibited a significant reduction in DYT1 MNs. By manipulating RANBP17 expression, we further demonstrated that RANBP17 plays an important role in facilitating the nuclear transport of both protein and transcript cargos in induced human neurons. Excitingly, the overexpression of RANBP17 emerged as a substantial mitigating factor, effectively restoring impaired NCT activity and rescuing neurodevelopmental deficits observed in DYT1 MNs. These findings shed light on the intricate molecular underpinnings of impaired NCT in DYT1 neurons and provide novel insights into the pathophysiology of DYT1 dystonia, potentially leading to the development of innovative treatment strategies.

Mechanical and chemical itch regulated by neuropeptide Y-Y1 signaling

Itch is a somatosensory sensation to remove potential harmful stimulation with a scratching desire, which could be divided into mechanical and chemical itch according to diverse stimuli, such as wool fiber and insect biting. It has been reported that neuropeptide Y (NPY) neurons, a population of spinal inhibitory interneurons, could gate the transmission of mechanical itch, with no effect on chemical itch. In our study, we verified that chemogenetic activation of NPY neurons could inhibit the mechanical itch as well as the chemical itch, which also attenuated the alloknesis phenomenon in the chronic dry skin model. Afterwards, intrathecal administration of NPY1R agonist, [Leu31, Pro34]-NPY (LP-NPY), showed the similar inhibition effect on mechanical itch, chemical itch and alloknesis as chemo-activation of NPY neurons. Whereas, intrathecal administration of NPY1R antagonist BIBO 3304 enhanced mechanical itch and reversed the alloknesis phenomenon inhibited by LP-NPY treatment. Moreover, selectively knocking down NPY1R by intrathecal injection of Npy1r siRNA enhanced mechanical and chemical itch behavior as well. These results indicate that NPY neurons in spinal cord regulate mechanical and chemical itch, and alloknesis in dry skin model through NPY1 receptors.

Molecular analyses of zebrafish V0v spinal interneurons and identification of transcriptional regulators downstream of Evx1 and Evx2 in these cells

BACKGROUND

V0v spinal interneurons are highly conserved, glutamatergic, commissural neurons that function in locomotor circuits. We have previously shown that Evx1 and Evx2 are required to specify the neurotransmitter phenotype of these cells. However, we still know very little about the gene regulatory networks that act downstream of these transcription factors in V0v cells.

METHODS

To identify candidate members of V0v gene regulatory networks, we FAC-sorted wild-type and evx1;evx2 double mutant zebrafish V0v spinal interneurons and expression-profiled them using microarrays and single cell RNA-seq. We also used in situ hybridization to compare expression of a subset of candidate genes in evx1;evx2 double mutants and wild-type siblings.

RESULTS

Our data reveal two molecularly distinct subtypes of zebrafish V0v spinal interneurons at 48 h and suggest that, by this stage of development, evx1;evx2 double mutant cells transfate into either inhibitory spinal interneurons, or motoneurons. Our results also identify 25 transcriptional regulator genes that require Evx1/2 for their expression in V0v interneurons, plus a further 11 transcriptional regulator genes that are repressed in V0v interneurons by Evx1/2. Two of the latter genes are hmx2 and hmx3a. Intriguingly, we show that Hmx2/3a, repress dI2 interneuron expression of skor1a and nefma, two genes that require Evx1/2 for their expression in V0v interneurons. This suggests that Evx1/2 might regulate skor1a and nefma expression in V0v interneurons by repressing Hmx2/3a expression.

CONCLUSIONS

This study identifies two molecularly distinct subsets of zebrafish V0v spinal interneurons, as well as multiple transcriptional regulators that are strong candidates for acting downstream of Evx1/2 to specify the essential functional characteristics of these cells. Our data further suggest that in the absence of both Evx1 and Evx2, V0v spinal interneurons initially change their neurotransmitter phenotypes from excitatory to inhibitory and then, later, start to express markers of distinct types of inhibitory spinal interneurons, or motoneurons. Taken together, our findings significantly increase our knowledge of V0v and spinal development and move us closer towards the essential goal of identifying the complete gene regulatory networks that specify this crucial cell type.

Molecular Analyses of V0v Spinal Interneurons and Identification of Transcriptional Regulators Downstream of Evx1 and Evx2 in these Cells

Background

V0v spinal interneurons are highly conserved, glutamatergic, commissural neurons that function in locomotor circuits. We have previously shown that Evx1 and Evx2 are required to specify the neurotransmitter phenotype of these cells. However, we still know very little about the gene regulatory networks that act downstream of these transcription factors in V0v cells.

Methods

To identify candidate members of V0v gene regulatory networks, we FAC-sorted WT and evx1;evx2 double mutant zebrafish V0v spinal interneurons and expression-profiled them using microarrays and single cell RNA-seq. We also used in situ hybridization to compare expression of a subset of candidate genes in evx1;evx2 double mutants and wild-type siblings.

Results

Our data reveal two molecularly distinct subtypes of V0v spinal interneurons at 48 h and suggest that, by this stage of development, evx1;evx2 double mutant cells transfate into either inhibitory spinal interneurons, or motoneurons. Our results also identify 25 transcriptional regulator genes that require Evx1/2 for their expression in V0v interneurons, plus a further 11 transcriptional regulator genes that are repressed in V0v interneurons by Evx1/2. Two of the latter genes are hmx2 and hmx3a. Intriguingly, we show that Hmx2/3a, repress dI2 interneuronal expression of skor1a and nefma, two genes that require Evx1/2 for their expression in V0v interneurons. This suggests that Evx1/2 might regulate skor1a and nefma expression in V0v interneurons by repressing Hmx2/3a expression.

Conclusions

This study identifies two molecularly distinct subsets of V0v spinal interneurons, as well as multiple transcriptional regulators that are strong candidates for acting downstream of Evx1/2 to specify the essential functional characteristics of these cells. Our data further suggest that in the absence of both Evx1 and Evx2, V0v spinal interneurons initially change their neurotransmitter phenotypes from excitatory to inhibitory and then, later, start to express markers of distinct types of inhibitory spinal interneurons, or motoneurons. Taken together, our findings significantly increase our knowledge of V0v and spinal development and move us closer towards the essential goal of identifying the complete gene regulatory networks that specify this crucial cell type.

Amplification of the PLAG-family genes-PLAGL1 and PLAGL2-is a key feature of the novel tumor type CNS embryonal tumor with PLAGL amplification

Pediatric central nervous system (CNS) tumors represent the most common cause of cancer-related death in children aged 0-14 years. They differ from their adult counterparts, showing extensive clinical and molecular heterogeneity as well as a challenging histopathological spectrum that often impairs accurate diagnosis. Here, we use DNA methylation-based CNS tumor classification in combination with copy number, RNA-seq, and ChIP-seq analysis to characterize a newly identified CNS tumor type. In addition, we report histology, patient characteristics, and survival data in this tumor type. We describe a biologically distinct pediatric CNS tumor type (n = 31 cases) that is characterized by focal high-level amplification and resultant overexpression of either PLAGL1 or PLAGL2, and an absence of recurrent genetic alterations characteristic of other pediatric CNS tumor types. Both genes act as transcription factors for a regulatory subset of imprinted genes (IGs), components of the Wnt/β-Catenin pathway, and the potential drug targets RET and CYP2W1, which are also specifically overexpressed in this tumor type. A derived PLAGL-specific gene expression signature indicates dysregulation of imprinting control and differentiation/development. These tumors occurred throughout the neuroaxis including the cerebral hemispheres, cerebellum, and brainstem, and were predominantly composed of primitive embryonal-like cells lacking robust expression of markers of glial or neuronal differentiation (e.g., GFAP, OLIG2, and synaptophysin). Tumors with PLAGL1 amplification were typically diagnosed during adolescence (median age 10.5 years), whereas those with PLAGL2 amplification were diagnosed during early childhood (median age 2 years). The 10-year overall survival was 66% for PLAGL1-amplified tumors, 25% for PLAGL2-amplified tumors, 18% for male patients, and 82% for female patients. In summary, we describe a new type of biologically distinct CNS tumor characterized by PLAGL1/2 amplification that occurs predominantly in infants and toddlers (PLAGL2) or adolescents (PLAGL1) which we consider best classified as a CNS embryonal tumor and which is associated with intermediate survival. The cell of origin and optimal treatment strategies remain to be defined.

Tlx3 Exerts Direct Control in Specifying Excitatory Over Inhibitory Neurons in the Dorsal Spinal Cord

The spinal cord dorsal horn is a major station for integration and relay of somatosensory information and comprises both excitatory and inhibitory neuronal populations. The homeobox gene Tlx3 acts as a selector gene to control the development of late-born excitatory (dILB) neurons by specifying glutamatergic transmitter fate in dorsal spinal cord. However, since Tlx3 direct transcriptional targets remain largely unknown, it remains to be uncovered how Tlx3 functions to promote excitatory cell fate. Here we combined a genomics approach based on chromatin immunoprecipitation followed by next generation sequencing (ChIP-seq) and expression profiling, with validation experiments in Tlx3 null embryos, to characterize the transcriptional program of Tlx3 in mouse embryonic dorsal spinal cord. We found most dILB neuron specific genes previously identified to be directly activated by Tlx3. Surprisingly, we found Tlx3 also directly represses many genes associated with the alternative inhibitory dILA neuronal fate. In both cases, direct targets include transcription factors and terminal differentiation genes, showing that Tlx3 directly controls cell identity at distinct levels. Our findings provide a molecular frame for the master regulatory role of Tlx3 in developing glutamatergic dILB neurons. In addition, they suggest a novel function for Tlx3 as direct repressor of GABAergic dILA identity, pointing to how generation of the two alternative cell fates being tightly coupled.

Neurog2 directly converts astrocytes into functional neurons in midbrain and spinal cord

Conversion of astrocytes into neurons in vivo offers an alternative therapeutic approach for neuronal loss after injury or disease. However, not only the efficiency of the conversion of astrocytes into functional neurons by single Neurog2, but also the conundrum that whether Neurog2-induced neuronal cells (Neurog2-iNs) are further functionally integrated into existing matured neural circuits remains unknown. Here, we adopted the AAV(2/8) delivery system to overexpress single factor Neurog2 into astrocytes and found that the majority of astrocytes were successfully converted into neuronal cells in multiple brain regions, including the midbrain and spinal cord. In the midbrain, Neurog2-induced neuronal cells (Neurog2-iNs) exhibit neuronal morphology, mature electrophysiological properties, glutamatergic identity (about 60%), and synapse-like configuration local circuits. In the spinal cord, astrocytes from both the intact and lesioned sources could be converted into functional neurons with ectopic expression of Neurog2 alone. Notably, further evidence from our study also proves that Neurog2-iNs in the intact spinal cord are capable of responding to diverse afferent inputs from dorsal root ganglion (DRG). Together, this study does not merely demonstrate the feasibility of Neurog2 for efficient in vivo reprogramming, it gives an indication for the Neurog2-iNs as a functional and potential factor in cell-replacement therapy.

Atxn2-CAG100-KnockIn mouse spinal cord shows progressive TDP43 pathology associated with cholesterol biosynthesis suppression

Large polyglutamine expansions in Ataxin-2 (ATXN2) cause multi-system nervous atrophy in Spinocerebellar Ataxia type 2 (SCA2). Intermediate size expansions carry a risk for selective motor neuron degeneration, known as Amyotrophic Lateral Sclerosis (ALS). Conversely, the depletion of ATXN2 prevents disease progression in ALS. Although ATXN2 interacts directly with RNA, and in ALS pathogenesis there is a crucial role of RNA toxicity, the affected functional pathways remain ill defined. Here, we examined an authentic SCA2 mouse model with Atxn2-CAG100-KnockIn for a first definition of molecular mechanisms in spinal cord pathology. Neurophysiology of lower limbs detected sensory neuropathy rather than motor denervation. Triple immunofluorescence demonstrated cytosolic ATXN2 aggregates sequestrating TDP43 and TIA1 from the nucleus. In immunoblots, this was accompanied by elevated CASP3, RIPK1 and PQBP1 abundance. RT-qPCR showed increase of Grn, Tlr7 and Rnaset2 mRNA versus Eif5a2, Dcp2, Uhmk1 and Kif5a decrease. These SCA2 findings overlap well with known ALS features. Similar to other ataxias and dystonias, decreased mRNA levels for Unc80, Tacr1, Gnal, Ano3, Kcna2, Elovl5 and Cdr1 contrasted with Gpnmb increase. Preterminal stage tissue showed strongly activated microglia containing ATXN2 aggregates, with parallel astrogliosis. Global transcriptome profiles from stages of incipient motor deficit versus preterminal age identified molecules with progressive downregulation, where a cluster of cholesterol biosynthesis enzymes including Dhcr24, Msmo1, Idi1 and Hmgcs1 was prominent. Gas chromatography demonstrated a massive loss of crucial cholesterol precursor metabolites. Overall, the ATXN2 protein aggregation process affects diverse subcellular compartments, in particular stress granules, endoplasmic reticulum and receptor tyrosine kinase signaling. These findings identify new targets and potential biomarkers for neuroprotective therapies.

The Role of PAX2 in Neurodevelopment and Disease

In developmental biology, transcription factors are involved in regulating the process of neural development, controlling the differentiation of nerve cells, and affecting the normal functioning of neural circuits. Transcription factors regulate the expression of multiple genes at the same time and have become a key gene category that is recognized to be disrupted in neurodevelopmental disorders such as autism spectrum disorders. This paper briefly introduces the expression and role of PAX2 in neurodevelopment and discusses the neurodevelopmental disorders associated with Pax2 mutations and its possible mechanism. Firstly, mutations in the human Pax2 gene are associated with abnormalities in multiple systems which can result in neurodevelopmental disorders such as intellectual disability, epilepsy and autism spectrum disorders. Secondly, the structure of Pax2 gene and PAX2 protein, as well as the function of Pax2 gene in neural development, was discussed. Finally, a diagram of the PAX2 protein regulatory network was made and a possible molecular mechanism of Pax2 mutations leading to neurodevelopmental disorders from the perspectives of developmental process and protein function was proposed.

Targeting spinal neuropeptide Y1 receptor-expressing interneurons to alleviate chronic pain and itch

An accelerating basic science literature is providing key insights into the mechanisms by which spinal neuropeptide Y (NPY) inhibits chronic pain. A key target of pain inhibition is the G_i-coupled neuropeptide Y1 receptor (Y1). Y1 is located in key sites of pain transmission, including the peptidergic subpopulation of primary afferent neurons and a dense subpopulation of small, excitatory, glutamatergic/somatostatinergic interneurons (Y1-INs) that are densely expressed in the dorsal horn, particularly in superficial lamina I-II. Selective ablation of spinal Y1-INs with an NPY-conjugated saporin neurotoxin attenuates the development of peripheral nerve injury-induced mechanical and cold hypersensitivity. Conversely, conditional knockdown of NPY expression or intrathecal administration of Y1 antagonists reinstates hypersensitivity in models of chronic latent pain sensitization. These and other results indicate that spinal NPY release and the consequent inhibition of pain facilitatory Y1-INs represent an important mechanism of endogenous analgesia. This mechanism can be mimicked with exogenous pharmacological approaches (e.g. intrathecal administration of Y1 agonists) to inhibit mechanical and thermal hypersensitivity and spinal neuron activity in rodent models of neuropathic, inflammatory, and postoperative pain. Pharmacological activation of Y1 also inhibits mechanical- and histamine-induced itch. These immunohistochemical, pharmacological, and cell type-directed lesioning data, in combination with recent transcriptomic findings, point to Y1-INs as a promising therapeutic target for the development of spinally directed NPY-Y1 agonists to treat both chronic pain and itch.

Transcriptomics Analysis of Porcine Caudal Dorsal Root Ganglia in Tail Amputated Pigs Shows Long-Term Effects on Many Pain-Associated Genes

Tail amputation by tail docking or as an extreme consequence of tail biting in commercial pig production potentially has serious implications for animal welfare. Tail amputation causes peripheral nerve injury that might be associated with lasting chronic pain. The aim of this study was to investigate the short- and long-term effects of tail amputation in pigs on caudal DRG gene expression at different stages of development, particularly in relation to genes associated with nociception and pain. Microarrays were used to analyse whole DRG transcriptomes from tail amputated and sham-treated pigs 1, 8, and 16 weeks following tail treatment at either 3 or 63 days of age (8 pigs/treatment/age/time after treatment; n = 96). Tail amputation induced marked changes in gene expression (up and down) compared to sham-treated intact controls for all treatment ages and time points after tail treatment. Sustained changes in gene expression in tail amputated pigs were still evident 4 months after tail injury. Gene correlation network analysis revealed two co-expression clusters associated with amputation: Cluster A (759 down-regulated) and Cluster B (273 up-regulated) genes. Gene ontology (GO) enrichment analysis identified 124 genes in Cluster A and 61 genes in Cluster B associated with both “inflammatory pain” and “neuropathic pain.” In Cluster A, gene family members of ion channels e.g., voltage-gated potassium channels (VGPC) and receptors e.g., GABA receptors, were significantly down-regulated compared to shams, both of which are linked to increased peripheral nerve excitability after axotomy. Up-regulated gene families in Cluster B were linked to transcriptional regulation, inflammation, tissue remodeling, and regulatory neuropeptide activity. These findings, demonstrate that tail amputation causes sustained transcriptomic expression changes in caudal DRG cells involved in inflammatory and neuropathic pain pathways.

Facilitation of neuropathic pain by the NPY Y1 receptor-expressing subpopulation of excitatory interneurons in the dorsal horn

Endogenous neuropeptide Y (NPY) exerts long-lasting spinal inhibitory control of neuropathic pain, but its mechanism of action is complicated by the expression of its receptors at multiple sites in the dorsal horn: NPY Y1 receptors (Y1Rs) on post-synaptic neurons and both Y1Rs and Y2Rs at the central terminals of primary afferents. We found that Y1R-expressing spinal neurons contain multiple markers of excitatory but not inhibitory interneurons in the rat superficial dorsal horn. To test the relevance of this spinal population to the development and/or maintenance of acute and neuropathic pain, we selectively ablated Y1R-expressing interneurons with intrathecal administration of an NPY-conjugated saporin ribosomal neurotoxin that spares the central terminals of primary afferents. NPY-saporin decreased spinal Y1R immunoreactivity but did not change the primary afferent terminal markers isolectin B4 or calcitonin-gene-related peptide immunoreactivity. In the spared nerve injury (SNI) model of neuropathic pain, NPY-saporin decreased mechanical and cold hypersensitivity, but disrupted neither normal mechanical or thermal thresholds, motor coordination, nor locomotor activity. We conclude that Y1R-expressing excitatory dorsal horn interneurons facilitate neuropathic pain hypersensitivity. Furthermore, this neuronal population remains sensitive to intrathecal NPY after nerve injury. This neuroanatomical and behavioral characterization of Y1R-expressing excitatory interneurons provides compelling evidence for the development of spinally-directed Y1R agonists to reduce chronic neuropathic pain.

Suppression of Pax2 Attenuates Allodynia and Hyperalgesia through ET-1-ETAR-NFAT5 Signaling in a Rat Model of Neuropathic Pain

Endothelin-1 (ET-1) and its receptors (ETAR/ETBR) emerge to be a key signaling axis in neuropathic pain processing and are recognized as new therapeutic targets. Yet, little is known on the functional regulation of ET-1 axis during neuropathic pain. Bioinformatics analysis indicated that paired box gene 2 (Pax2) or nuclear factor of activated T-cells 5 (NFAT5), two transcription factors involved in the modulation of neurotransmission, may regulate ET-1. Therefore, we hypothesized that ET-1 axis may be regulated by Pax2 or NFAT5 in the development of neuropathic pain. After partial sciatic nerve ligation (pSNL), rats displayed allodynia and hyperalgesia, which was associated with increased mRNA and protein expressions of spinal Pax2, NFAT5, and mRNA levels of ET-1 and ETAR, but not ETBR. Knockdown of Pax2 or NFAT5 with siRNA, or inhibition of ETAR with BQ-123 attenuated pSNL-induced pain-like behaviors. At molecular level, Pax2 siRNA, but not NFAT5 siRNA, downregulated ET-1 and ETAR, while ETAR inhibitor reduced NFAT5, indicating Pax2 in the upstream of ET-1 axis with NFAT5 in the downstream. Further, suppression of Pax2 (inhibiting ET-1) or impairment of ET-1 signaling (inhibition of ETAR and/or decrease of NFAT5) deactivated mitogen-activated protein kinases (MAPK) and nuclear factor-kappa B (NF-κB) signaling pathways, supporting the significance of functional regulation of ET-1 axis in neuropathic pain signaling. These findings demonstrate that Pax2 targeting ET-1-ETAR-NFAT5 is a novel regulatory mechanism underlying neuropathic pain.

Transcriptional Profiling of Somatostatin Interneurons in the Spinal Dorsal Horn

The spinal dorsal horn (SDH) is comprised of distinct neuronal populations that process different somatosensory modalities. Somatostatin (SST)-expressing interneurons in the SDH have been implicated specifically in mediating mechanical pain. Identifying the transcriptomic profile of SST neurons could elucidate the unique genetic features of this population and enable selective analgesic targeting. To that end, we combined the Isolation of Nuclei Tagged in Specific Cell Types (INTACT) method and Fluorescence Activated Nuclei Sorting (FANS) to capture tagged SST nuclei in the SDH of adult male mice. Using RNA-sequencing (RNA-seq), we uncovered more than 13,000 genes. Differential gene expression analysis revealed more than 900 genes with at least 2-fold enrichment. In addition to many known dorsal horn genes, we identified and validated several novel transcripts from pharmacologically tractable functional classes: Carbonic Anhydrase 12 (Car12), Phosphodiesterase 11 A (Pde11a), and Protease-Activated Receptor 3 (F2rl2). In situ hybridization of these novel genes showed differential expression patterns in the SDH, demonstrating the presence of transcriptionally distinct subpopulations within the SST population. Overall, our findings provide new insights into the gene repertoire of SST dorsal horn neurons and reveal several novel targets for pharmacological modulation of this pain-mediating population and treatment of pathological pain.

Preprotachykinin A is expressed by a distinct population of excitatory neurons in the mouse superficial spinal dorsal horn including cells that respond to noxious and pruritic stimuli

Expression of the substance P precursor preprotachykinin A defines a distinct population of superficial dorsal horn excitatory neurons, many of which respond to noxious or pruritic stimuli.

The superficial dorsal horn, which is the main target for nociceptive and pruritoceptive primary afferents, contains a high density of excitatory interneurons. Our understanding of their roles in somatosensory processing has been restricted by the difficulty of distinguishing functional populations among these cells. We recently defined 3 nonoverlapping populations among the excitatory neurons, based on the expression of neurotensin, neurokinin B, and gastrin-releasing peptide. Here we identify and characterise another population: neurons that express the tachykinin peptide substance P. We show with immunocytochemistry that its precursor protein (preprotachykinin A, PPTA) can be detected in ∼14% of lamina I-II neurons, and these are concentrated in the outer part of lamina II. Over 80% of the PPTA-positive cells lack the transcription factor Pax2 (which determines an inhibitory phenotype), and these account for ∼15% of the excitatory neurons in this region. They are different from the neurotensin, neurokinin B, or gastrin-releasing peptide neurons, although many of them contain somatostatin, which is widely expressed among superficial dorsal horn excitatory interneurons. We show that many of these cells respond to noxious thermal and mechanical stimuli and to intradermal injection of pruritogens. Finally, we demonstrate that these cells can also be identified in a knock-in Cre mouse line (Tac1^Cre), although our findings suggest that there is an additional population of neurons that transiently express PPTA. This population of substance P–expressing excitatory neurons is likely to play an important role in the transmission of signals that are perceived as pain and itch.

Spinal Circuits for Touch, Pain, and Itch

The exteroceptive somatosensory system is important for reflexive and adaptive behaviors and for the dynamic control of movement in response to external stimuli. This review outlines recent efforts using genetic approaches in the mouse to map the spinal cord circuits that transmit and gate the cutaneous somatosensory modalities of touch, pain, and itch. Recent studies have revealed an underlying modular architecture in which nociceptive, pruritic, and innocuous stimuli are processed by distinct molecularly defined interneuron cell types. These include excitatory populations that transmit information about both innocuous and painful touch and inhibitory populations that serve as a gate to prevent innocuous stimuli from activating the nociceptive and pruritic transmission pathways. By dissecting the cellular composition of dorsal-horn networks, studies are beginning to elucidate the intricate computational logic of somatosensory transformation in health and disease.

Autism and intellectual disability in a patient with two microdeletions in 6q16: a contiguous gene deletion syndrome?

Background

Copy number variations play a significant role in the aetiology of developmental disabilities including non-syndromic intellectual disability and autism.

Case presentation

We describe a 19-year old patient with intellectual disability and autism for whom chromosomal microarray (CMA) analysis showed the unusual finding of two de novo microdeletions in cis position on chromosome 6q16.1q16.2 and 6q16.3. The two deletions span 10 genes, including FBXL4, POU3F2, PRDM13, CCNC, COQ3 and GRIK2. We compared phenotypes of patients with similar deletions and looked at the involvement of the genes in neuronal networks in order to determine the pathogenicity of our patient’s deletions.

Conclusions

We suggest that both deletions on 6q are causing his disease phenotype since they harbour several genes which are implicated in pathways of neuronal development and function. Further studies regarding the interaction between PRDM13 and GRIK2 specifically may be interesting.

Neuronal networks and nociceptive processing in the dorsal horn of the spinal cord

The dorsal horn (DH) of the spinal cord receives a variety of sensory information arising from the inner and outer environment, as well as modulatory inputs from supraspinal centers. This information is integrated by the DH before being forwarded to brain areas where it may lead to pain perception. Spinal integration of this information relies on the interplay between different DH neurons forming complex and plastic neuronal networks. Elements of these networks are therefore potential targets for new analgesics and pain-relieving strategies. The present review aims at providing an overview of the current knowledge on these networks, with a special emphasis on those involving interlaminar communication in both physiological and pathological conditions.

Zinc finger transcription factor Casz1 expression is regulated by homeodomain transcription factor Prrxl1 in embryonic spinal dorsal horn late-born excitatory interneurons

The transcription factor Casz1 is required for proper assembly of vertebrate vasculature and heart morphogenesis as well as for temporal control of Drosophila neuroblasts and mouse retina progenitors in the generation of different cell types. Although Casz1 function in the mammalian nervous system remains largely unexplored, Casz1 is expressed in several regions of this system. Here we provide a detailed spatiotemporal characterization of Casz1 expression along mouse dorsal root ganglion (DRG) and dorsal spinal cord development by immunochemistry. In the DRG, Casz1 is broadly expressed in sensory neurons since they are born until perinatal age. In the dorsal spinal cord, Casz1 displays a more dynamic pattern being first expressed in dorsal interneuron 1 (dI1) progenitors and their derived neurons and then in a large subset of embryonic dorsal late-born excitatory (dILB) neurons that narrows gradually to become restricted perinatally to the inner portion. Strikingly, expression analyses using Prrxl1-knockout mice revealed that Prrxl1, a key transcription factor in the differentiation of dILB neurons, is a positive regulator of Casz1 expression in the embryonic dorsal spinal cord but not in the DRG. By performing chromatin immunoprecipitation in the dorsal spinal cord, we identified two Prrxl1-bound regions within Casz1 introns, suggesting that Prrxl1 directly regulates Casz1 transcription. Our work reveals that Casz1 lies downstream of Prrxl1 in the differentiation pathway of a large subset of dILB neurons and provides a framework for further studies of Casz1 in assembly of the DRG-spinal circuit.

Development of regional specificity of spinal and medullary dorsal horn neurons

Extensive studies have focused on the development and regionalization of neurons in the central nervous system (CNS). Many genes, which play crucial roles in the development of CNS neurons, have been identified. By using the technique “direct reprogramming”, neurons can be produced from multiple cell sources such as fibroblasts. However, understanding the region-specific regulation of neurons in the CNS is still one of the biggest challenges in the research field of neuroscience. Neurons located in the trigeminal subnucleus caudalis (Vc) and in the spinal dorsal horn (SDH) play crucial roles in pain and sensorimotor functions in the orofacial and other somatic body regions, respectively. Anatomically, Vc represents the most caudal component of the trigeminal system, and is contiguous with SDH. This review is focused on recent data dealing with the regional specificity involved in the development of neurons in Vc and SDH.

Evx1 and Evx2 specify excitatory neurotransmitter fates and suppress inhibitory fates through a Pax2-independent mechanism

BACKGROUND

For neurons to function correctly in neuronal circuitry they must utilize appropriate neurotransmitters. However, even though neurotransmitter specificity is one of the most important and defining properties of a neuron we still do not fully understand how neurotransmitter fates are specified during development. Most neuronal properties are determined by the transcription factors that neurons express as they start to differentiate. While we know a few transcription factors that specify the neurotransmitter fates of particular neurons, there are still many spinal neurons for which the transcription factors specifying this critical phenotype are unknown. Strikingly, all of the transcription factors that have been identified so far as specifying inhibitory fates in the spinal cord act through Pax2. Even Tlx1 and Tlx3, which specify the excitatory fates of dI3 and dI5 spinal neurons work at least in part by down-regulating Pax2.

METHODS

In this paper we use single and double mutant zebrafish embryos to identify the spinal cord functions of Evx1 and Evx2.

RESULTS

We demonstrate that Evx1 and Evx2 are expressed by spinal cord V0v cells and we show that these cells develop into excitatory (glutamatergic) Commissural Ascending (CoSA) interneurons. In the absence of both Evx1 and Evx2, V0v cells still form and develop a CoSA morphology. However, they lose their excitatory fate and instead express markers of a glycinergic fate. Interestingly, they do not express Pax2, suggesting that they are acquiring their inhibitory fate through a novel Pax2-independent mechanism.

CONCLUSIONS

Evx1 and Evx2 are required, partially redundantly, for spinal cord V0v cells to become excitatory (glutamatergic) interneurons. These results significantly increase our understanding of the mechanisms of neuronal specification and the genetic networks involved in these processes.

Ascl1 Converts Dorsal Midbrain Astrocytes into Functional Neurons In Vivo

In vivo induction of non-neuronal cells into neurons by transcription factors offers potential therapeutic approaches for neural regeneration. Although generation of induced neuronal (iN) cells in vitro and in vivo has been reported, whether iN cells can be fully integrated into existing circuits remains unclear. Here we show that expression of achaete-scute complex homolog-like 1 (Ascl1) alone is sufficient to convert dorsal midbrain astrocytes of mice into functional iN cells in vitro and in vivo. Specific expression of Ascl1 in astrocytes by infection with GFAP-adeno-associated virus (AAV) vector converts astrocytes in dorsal midbrain, striatum, and somatosensory cortex of postnatal and adult mice into functional neurons in vivo. These iN cells mature progressively, exhibiting neuronal morphology and markers, action potentials, and synaptic inputs from and output to existing neurons. Thus, a single transcription factor, Ascl1, is sufficient to convert brain astrocytes into functional neurons, and GFAP-AAV is an efficient vector for generating iN cells from astrocytes in vivo.

The generation of knock-in mice expressing fluorescently tagged galanin receptors 1 and 2

The neuropeptide galanin has diverse roles in the central and peripheral nervous systems, by activating the G protein-coupled receptors Gal1, Gal2 and the less studied Gal3 (GalR1-3 gene products). There is a wealth of data on expression of Gal1-3 at the mRNA level, but not at the protein level due to the lack of specificity of currently available antibodies. Here we report the generation of knock-in mice expressing Gal1 or Gal2 receptor fluorescently tagged at the C-terminus with, respectively, mCherry or hrGFP (humanized Renilla green fluorescent protein). In dorsal root ganglia (DRG) neurons expressing the highest levels of Gal1-mCherry, localization to the somatic cell membrane was detected by live-cell fluorescence and immunohistochemistry, and that fluorescence decreased upon addition of galanin. In spinal cord, abundant Gal1-mCherry immunoreactive processes were detected in the superficial layers of the dorsal horn, and highly expressing intrinsic neurons of the lamina III/IV border showed both somatic cell membrane localization and outward transport of receptor from the cell body, detected as puncta within cell processes. In brain, high levels of Gal1-mCherry immunofluorescence were detected within thalamus, hypothalamus and amygdala, with a high density of nerve endings in the external zone of the median eminence, and regions with lesser immunoreactivity included the dorsal raphe nucleus. Gal2-hrGFP mRNA was detected in DRG, but live-cell fluorescence was at the limits of detection, drawing attention to both the much lower mRNA expression than to Gal1 in mice and the previously unrecognized potential for translational control by upstream open reading frames (uORFs).

Tlx3 promotes glutamatergic neuronal subtype specification through direct interactions with the chromatin modifier CBP

Nervous system development relies on the generation of precise numbers of excitatory and inhibitory neurons. The homeodomain transcription factor, T-cell leukemia 3 (Tlx3), functions as the master neuronal fate regulator by instructively promoting the specification of glutamatergic excitatory neurons and suppressing the specification of gamma-aminobutyric acid (GABAergic) neurons. However, how Tlx3 promotes glutamatergic neuronal subtype specification is poorly understood. In this study, we found that Tlx3 directly interacts with the epigenetic co-activator cyclic adenosine monophosphate (cAMP)-response element-binding protein (CREB)-binding protein (CBP) and that the Tlx3 homeodomain is essential for this interaction. The interaction between Tlx3 and CBP was enhanced by the three amino acid loop extension (TALE)-class homeodomain transcription factor, pre-B-cell leukemia transcription factor 3 (Pbx3). Using mouse embryonic stem (ES) cells stably expressing Tlx3, we found that the interaction between Tlx3 and CBP became detectable only after these Tlx3-expressing ES cells were committed to a neural lineage, which coincided with increased Pbx3 expression during neural differentiation from ES cells. Forced expression of mutated Tlx3 lacking the homeodomain in ES cells undergoing neural differentiation resulted in significantly reduced expression of glutamatergic neuronal subtype markers, but had little effect on the expression on pan neural markers. Collectively, our results strongly suggest that functional interplay between Tlx3 and CBP plays a critical role in neuronal subtype specification, providing novel insights into the epigenetic regulatory mechanism that modulates the transcriptional efficacy of a selective set of neuronal subtype-specific genes during differentiation.

Control of axon guidance and neurotransmitter phenotype of dB1 hindbrain interneurons by Lim-HD code

Hindbrain dorsal interneurons (HDIs) are implicated in receiving, processing, integrating, and transmitting sensory inputs from the periphery and spinal cord, including the vestibular, auditory, and proprioceptive systems. During development, multiple molecularly defined HDI types are set in columns along the dorsoventral axis, before migrating along well-defined trajectories to generate various brainstem nuclei. Major brainstem functions rely on the precise assembly of different interneuron groups and higher brain domains into common circuitries. Yet, knowledge regarding interneuron axonal patterns, synaptic targets, and the transcriptional control that govern their connectivity is sparse. The dB1 class of HDIs is formed in a district dorsomedial position along the hindbrain and gives rise to the inferior olive nuclei, dorsal cochlear nuclei, and vestibular nuclei. dB1 interneurons express various transcription factors (TFs): the pancreatic transcription factor 1a (Ptf1a), the homeobox TF-Lbx1 and the Lim-homeodomain (Lim-HD), and TF Lhx1 and Lhx5. To decipher the axonal and synaptic connectivity of dB1 cells, we have used advanced enhancer tools combined with conditional expression systems and the PiggyBac-mediated DNA transposition system in avian embryos. Multiple ipsilateral and contralateral axonal projections were identified ascending toward higher brain centers, where they formed synapses in the Purkinje cerebellar layer as well as at discrete midbrain auditory and vestibular centers. Decoding the mechanisms that instruct dB1 circuit formation revealed a fundamental role for Lim-HD proteins in regulating their axonal patterns, synaptic targets, and neurotransmitter choice. Together, this study provides new insights into the assembly and heterogeneity of HDIs connectivity and its establishment through the central action of Lim-HD governed programs.

Normal and abnormal coding of somatosensory stimuli causing pain

Noxious stimuli usually cause pain and pain usually arises from noxious stimuli, but exceptions to these apparent truisms are the basis for clinically important problems and provide valuable insight into the neural code for pain. In this Review, we discuss how painful sensations arise. We argue that, although primary somatosensory afferents are tuned to specific stimulus features, natural stimuli often activate more than one type of afferent. Manipulating coactivation patterns can alter perception in ways that argue against each type of afferent acting independently (as expected for strictly labeled lines), suggesting instead that signals conveyed by different types of afferents interact. Deciphering the central circuits that mediate those interactions is critical for explaining the generation and modulation of neural signals that ultimately elicit pain. The advent of genetic and optical dissection techniques promise to dramatically accelerate progress toward this goal, which will facilitate the rational design of future pain therapeutics.

Ontogeny of excitatory spinal neurons processing distinct somatic sensory modalities

Spatial and temporal cues govern the genesis of a diverse array of neurons located in the dorsal spinal cord, including dI1-dI6, dIL(A), and dIL(B) subtypes, but their physiological functions are poorly understood. Here we generated a new line of conditional knock-out (CKO) mice, in which the homeobox gene Tlx3 was removed in dI5 and dIL(B) cells. In these CKO mice, development of a subset of excitatory neurons located in laminae I and II was impaired, including itch-related GRPR-expressing neurons, PKCγ-expressing neurons, and neurons expressing three neuropeptide genes: somatostatin, preprotachykinin 1, and the gastrin-releasing peptide. These CKO mice displayed marked deficits in generating nocifensive motor behaviors evoked by a range of pain-related or itch-related stimuli. The mutants also failed to exhibit escape response evoked by dynamic mechanical stimuli but retained the ability to sense innocuous cooling and/or warm. Thus, our studies provide new insight into the ontogeny of spinal neurons processing distinct sensory modalities.

Tlx3 controls cholinergic transmitter and Peptide phenotypes in a subset of prenatal sympathetic neurons

The embryonic sympathetic nervous system consists of predominantly noradrenergic neurons and a very small population of cholinergic neurons. Postnatal development further allows target-dependent switch of a subset of noradrenergic neurons into cholinergic phenotype. How embryonic cholinergic neurons are specified at the prenatal stages remains largely unknown. In this study, we found that the expression of transcription factor Tlx3 was progressively restricted to a small population of embryonic sympathetic neurons in mice. Immunostaining for vesicular acetylcholine transporter (VAChT) showed that Tlx3 was highly expressed in cholinergic neurons at the late embryonic stage E18.5. Deletion of Tlx3 resulted in the loss of Vacht expression at E18.5 but not E12.5. By contrast, Tlx3 was required for expression of the cholinergic peptide vasoactive intestinal polypeptide (VIP), and somatostatin (SOM) at both E12.5 and E18.5. Furthermore, we found that, at E18.5 these putative cholinergic neurons expressed glial cell line-derived neurotrophic factor family coreceptor Ret but not tyrosine hydroxylase (Ret(+)/TH(-)). Deletion of Tlx3 also resulted in disappearance of high-level Ret expression. Last, unlike Tlx3, Ret was required for the expression of VIP and SOM at E18.5 but not E12.5. Together, these results indicate that transcription factor Tlx3 is required for the acquisition of cholinergic phenotype at the late embryonic stage as well as the expression and maintenance of cholinergic peptides VIP and SOM throughout prenatal development of mouse sympathetic neurons.

The homeodomain factor Gbx1 is required for locomotion and cell specification in the dorsal spinal cord

Dorsal horn neurons in the spinal cord integrate and relay sensory information to higher brain centers. These neurons are organized in specific laminae and different transcription factors are involved in their specification. The murine homeodomain Gbx1 protein is expressed in the mantle zone of the spinal cord at E12.5-13.5, correlating with the appearance of a discernable dorsal horn around E14 and eventually defining a narrow layer in the dorsal horn around perinatal stages. At postnatal stages, Gbx1 identifies a specific subpopulation of GABAergic neurons in the dorsal spinal cord. We have generated a loss of function mutation for Gbx1 and analyzed its consequences during spinal cord development. Gbx1^−/− mice are viable and can reproduce as homozygous null mutants. However, the adult mutant mice display an altered gait during forward movement that specifically affects the hindlimbs. This abnormal gait was evaluated by a series of behavioral tests, indicating that locomotion is impaired, but not muscle strength or motor coordination. Molecular analysis showed that the development of the dorsal horn is not profoundly affected in Gbx1^−/− mutant mice. However, analysis of terminal neuronal differentiation revealed that the proportion of GABAergic inhibitory interneurons in the superficial dorsal horn is diminished. Our study unveiled a role for Gbx1 in specifying a subset of GABAergic neurons in the dorsal horn of the spinal cord involved in the control of posterior limb movement.