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1.  Identification of spinal circuits transmitting and gating mechanical pain 
Cell  2014;159(6):1417-1432.
SUMMARY
Pain processing in the spinal cord has been postulated to rely on nociceptive transmission (T) neurons receiving inputs from nociceptors and Aβ mechanoreceptors, with Aβ inputs gated through feed-forward activation of spinal inhibitory neurons (IN). Here we used intersectional genetic manipulations to identify these critical components of pain transduction. Marking and ablating six populations of spinal excitatory and inhibitory neurons, coupled with behavioral and electrophysiological analysis, showed that excitatory neurons expressing somatostatin (SOM) represent T-type cells, whose ablation causes loss of mechanical pain. Inhibitory neurons marked by the expression of dynorphin (Dyn) represent IN-type neurons, which are necessary to gate Aβ fibers from activating SOM+ neurons to evoke pain. Therefore, peripheral mechanical nociceptors and Aβ mechanoreceptors, together with spinal SOM+ excitatory and Dyn+ inhibitory neurons form a microcircuit that transmits and gates mechanical pain.
doi:10.1016/j.cell.2014.11.003
PMCID: PMC4258511  PMID: 25467445
2.  Incoherent Feed-Forward Regulatory Loops Control Segregation of C-Mechanoreceptors, Nociceptors, and Pruriceptors 
The Journal of Neuroscience  2015;35(13):5317-5329.
Mammalian skin is innervated by diverse, unmyelinated C fibers that are associated with senses of pain, itch, temperature, or touch. A key developmental question is how this neuronal cell diversity is generated during development. We reported previously that the runt domain transcription factor Runx1 is required to coordinate the development of these unmyelinated cutaneous sensory neurons, including VGLUT3+ low-threshold c-mechanoreceptors (CLTMs), MrgprD+ polymodal nociceptors, MrgprA3+ pruriceptors, MrgprB4+ c-mechanoreceptors, and others. However, how these Runx1-dependent cutaneous sensory neurons are further segregated is poorly illustrated. Here, we find that the Runx1-dependent transcription factor gene Zfp521 is expressed in, and required for establishing molecular features that define, VGLUT3+ CLTMs. Furthermore, Runx1 and Zfp521 form a classic incoherent feedforward loop (I-FFL) in controlling molecular identities that normally belong to MrgprD+ neurons, with Runx1 and Zfp51 playing activator and repressor roles, respectively (in genetic terms). A knock-out of Zfp521 allows prospective VGLUT3 lineage neurons to acquire MrgprD+ neuron identities. Furthermore, Runx1 might form other I-FFLs to regulate the expression of MrgprA3 and MrgprB4, a mechanism preventing these genes from being expressed in Runx1-persistent VGLUT3+ and MrgprD+ neurons. The evolvement of these I-FFLs provides an explanation for how modality-selective sensory subtypes are formed during development and may also have intriguing implications for sensory neuron evolution and sensory coding.
doi:10.1523/JNEUROSCI.0122-15.2015
PMCID: PMC4381003  PMID: 25834056
low-threshold c-mechanoreceptors; nociceptors; pruriceptors; Runx1; sensory subtype specification; Zfp521
4.  Transcriptional profiling at whole population and single cell levels reveals somatosensory neuron molecular diversity 
eLife  null;3:e04660.
The somatosensory nervous system is critical for the organism's ability to respond to mechanical, thermal, and nociceptive stimuli. Somatosensory neurons are functionally and anatomically diverse but their molecular profiles are not well-defined. Here, we used transcriptional profiling to analyze the detailed molecular signatures of dorsal root ganglion (DRG) sensory neurons. We used two mouse reporter lines and surface IB4 labeling to purify three major non-overlapping classes of neurons: 1) IB4+SNS-Cre/TdTomato+, 2) IB4−SNS-Cre/TdTomato+, and 3) Parv-Cre/TdTomato+ cells, encompassing the majority of nociceptive, pruriceptive, and proprioceptive neurons. These neurons displayed distinct expression patterns of ion channels, transcription factors, and GPCRs. Highly parallel qRT-PCR analysis of 334 single neurons selected by membership of the three populations demonstrated further diversity, with unbiased clustering analysis identifying six distinct subgroups. These data significantly increase our knowledge of the molecular identities of known DRG populations and uncover potentially novel subsets, revealing the complexity and diversity of those neurons underlying somatosensation.
DOI: http://dx.doi.org/10.7554/eLife.04660.001
eLife digest
In the nervous system, a network of specialized neurons—known as the somatosensory system—carries information about sensations including touch, muscle position, temperature and pain. Distinct sets of somatosensory neurons are thought to carry information about the different types of sensations. In young animals, the precise switching on, or ‘expression’, of genes controls the formation of the network of neurons. However, it is not known exactly which genes are expressed in what types of neurons, where, or when.
Here, Chiu et al. used a technique called flow cytometry using different fluorescent markers to isolate a group of cells called Dorsal Root Ganglion (DRG) neurons in mice. These neurons have long thread-like fibers that extend from the spinal cord to the skin, muscles and joints all over the body. These fibers carry sensory information to the spinal cord, where it can be relayed to the brain and processed. The experiments compared three distinct types of DRG neuron and found that they differed in their ability to send information to other cells.
Chiu et al. analyzed the expression of all the genes in the three types of DRG neurons. Each type of neuron had distinct groups of genes that were being expressed. Also, several genes that are known to be important for sensation were expressed at different levels in the different types of cells. Next, large numbers of single cells were analyzed to find out the finer details about the three types of neuron. These findings made it possible to further divide the DRG neurons into six distinct subsets that matched previously known groups of somatosensory neurons, and also identified new ones.
Chiu et al.'s findings reveal the complexity and diversity of the neurons involved in carrying information about sensations towards the brain. This is an important step in classifying the nervous system, and uncovers many genes previously not linked to sensation. The next challenges lie in understanding how the expression of these genes in each type of neuron relates to their unique roles.
DOI: http://dx.doi.org/10.7554/eLife.04660.002
doi:10.7554/eLife.04660
PMCID: PMC4383053  PMID: 25525749
transcriptome; peripheral nervous system; somatosensation; DRG; nociception; proprioception; mouse
5.  Genetic control of the segregation of pain-related sensory neurons innervating the cutaneous versus deep tissues 
Cell reports  2013;5(5):1353-1364.
Summary
Mammalian pain-related sensory neurons are derived from TrkA lineage neurons located in the dorsal root ganglion. These neurons project to peripheral targets throughout the body, which can be divided into superficial and deep tissues. Here we find that the transcription factor Runx1 is required for the development of many epidermis-projecting TrkA lineage neurons. Accordingly, knockout of Runx1 leads to the selective loss of sensory innervation to the epidermis, whereas deep tissue innervation and two types of deep tissue pain are unaffected. Within these cutaneous neurons, Runx1 concurrently suppresses a large molecular program normally associated with sensory neurons that innervate deep tissues, such as muscle and visceral organs. Ectopic expression of Runx1 in these deep sensory neurons causes a loss of this molecular program and marked deficits in deep tissue pain. Thus this study provides new insight into a genetic program controlling the segregation of cutaneous versus deep tissue pain pathways.
doi:10.1016/j.celrep.2013.11.005
PMCID: PMC3895930  PMID: 24316076
6.  Dynamic Expression of Secreted Frizzled-related Protein 3 (sFRP3) in the developing mouse spinal cord and dorsal root ganglia 
Neuroscience  2013;0:594-601.
Wnt proteins have been implicated in regulating a variety of developmental processes in the central nervous system (CNS). Secreted Frizzled-related protein 3 (sFRP3) is a member of the sFRP family that can inhibit the Wnt signaling by binding directly to Wnts via their regions of homology to the Wnt-binding domain of Frizzleds. Recent studies suggested that sFRP3 plays an important role in cell proliferation and differentiation in various tissues. To understand the role of sFRP3 in neural development, we carried out detailed studies on the expression of sFRP3 in the developing nervous system. Our results revealed that sFRP3 is initially expressed in the ventricular zone of spinal cord and dorsal root ganglia (DRG), and later in the dorsal horn of spinal cord and subpopulation of DRG neurons. The spatiotemporally dynamic expression of sFRP3 strongly suggests that sFRP3 has potential functions in the sensory neuron genesis and sensory circuitry formation.
doi:10.1016/j.neuroscience.2013.06.044
PMCID: PMC3844105  PMID: 23827310
secreted frizzled-related protein 3; spinal cord; dorsal root ganglia; sensory circuit
7.  Normal and abnormal coding of painful sensations 
Nature neuroscience  2014;17(2):183-191.
Noxious stimuli cause pain and pain arises from noxious stimuli… usually. Exceptions to these apparent truisms are the basis for clinically important problems and provide valuable insight into the neural code for pain. In this Perspective, we will discuss how painful sensations are encoded. We will argue that although primary somatosensory afferents are specialized (i.e. tuned to specific stimulus features), natural stimuli often activate >1 type of afferent. Manipulating co-activation patterns can alter perception, which argues against each type of afferent acting independently (as expected for strictly labeled lines) and suggests 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 ultimately perceived as pain. The advent of genetic and optical dissection techniques promise to dramatically accelerate progress towards this goal, which will facilitate the rational design of future pain therapeutics.
doi:10.1038/nn.3629
PMCID: PMC4079041  PMID: 24473266
8.  Ontogeny of Excitatory Spinal Neurons Processing Distinct Somatic Sensory Modalities 
The Journal of Neuroscience  2013;33(37):14738-14748.
Spatial and temporal cues govern the genesis of a diverse array of neurons located in the dorsal spinal cord, including dI1-dI6, dILA, and dILB 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 dILB 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.
doi:10.1523/JNEUROSCI.5512-12.2013
PMCID: PMC3771039  PMID: 24027274
9.  Activity-dependent silencing reveals functionally distinct itch-generating sensory neurons 
Nature neuroscience  2013;16(7):910-918.
The peripheral terminals of primary sensory neurons detect histamine and non-histamine itch-provoking ligands through molecularly distinct transduction mechanisms. It remains unclear, however, whether these distinct pruritogens activate the same or different afferent fibers. We utilized a strategy of reversibly silencing specific subsets of murine pruritogen-sensitive sensory axons by targeted delivery of a charged sodium-channel blocker and found that functional blockade of histamine itch did not affect the itch evoked by chloroquine or SLIGRL-NH2, and vice versa. Notably, blocking itch-generating fibers did not reduce pain-associated behavior. However, silencing TRPV1+ or TRPA1+ neurons allowed AITC or capsaicin respectively to evoke itch, implying that certain peripheral afferents may normally indirectly inhibit algogens from eliciting itch. These findings support the presence of functionally distinct sets of itch-generating neurons and suggest that targeted silencing of activated sensory fibers may represent a clinically useful anti-pruritic therapeutic approach for histaminergic and non-histaminergic pruritus.
doi:10.1038/nn.3404
PMCID: PMC3695070  PMID: 23685721
10.  A genome-scale study of transcription factor expression in the branching mouse lung 
Background
Mammalian lung development consists of a series of precisely choreographed events that drive the progression from simple lung buds to the elaborately branched organ that fulfills the vital function of gas exchange. Strict transcriptional control is essential for lung development. Among the large number of transcription factors encoded in the mouse genome, only a small portion of them are known to be expressed and function in the developing lung. Thus a systematic investigation of transcription factors expressed in the lung is warranted.
Results
To enrich for genes that may be responsible for regional growth and patterning, we performed a screen using RNA in situ hybridization to identify genes that show restricted expression patterns in the embryonic lung. We focused on the pseudoglandular stage during which the lung undergoes branching morphogenesis, a cardinal event of lung development. Using a genome-scale probe set that represents over 90% of the transcription factors encoded in the mouse genome, we identified sixty-two transcription factor genes with localized expression in the epithelium, mesenchyme or both. Many of these genes have not been previously implicated in lung development.
Conclusions
Our findings provide new starting points for the elucidation of the transcriptional circuitry that controls lung development.
doi:10.1002/dvdy.23823
PMCID: PMC3529173  PMID: 22711520
lung; mouse; transcription factors; expression patterns; branching
11.  Runx1 Controls Terminal Morphology and Mechanosensitivity of VGLUT3-expressing C-Mechanoreceptors 
VGLUT3-expressing unmyelinated low-threshold mechanoreceptors (C-LTMRs) are proposed to mediate pleasant touch and/or pain, but the molecular programs controlling C-LTMR development are unknown. Here we performed genetic fate mapping, showing that VGLUT3 lineage sensory neurons are divided into two groups, based on transient or persistent VGLUT3 expression. VGLUT3-trasient neurons are large- or medium-diameter myelinated mechanoreceptors that form the Merkel cell-neurite complex. VGLUT3-persistent neurons are small-diameter unmyelinated neurons that are further divided into two subtypes: 1) tyrosine hydroxylase (TH)-positive C-LTMRs that form the longitudinal lanceolate endings around hairs, and 2) TH-negative neurons that form epidermal free nerve endings. We then found that VGLUT3-persistent neurons express the runt domain transcription factor Runx1. Analyses of mice with a conditional knockout of Runx1 in VGLUT3 lineage neurons demonstrate that Runx1 is pivotal to the development of VGLUT3-persistent neurons, such as the expression of VGLUT3 and TH and the formation of the longitudinal lanceolate endings. Furthermore, Runx1 is required to establish mechanosensitivity in C-LTMRs, by controlling the expression of the mechanically gated ion channel Piezo2. Surprisingly, both acute and chronic mechanical pain was largely unaffected in these Runx1 mutants. These findings appear to argue against the recently proposed role of VGLUT3 in C-LTMRs in mediating mechanical hypersensitivity induced by nerve injury or inflammation. Thus, our studies provide new insight into the genetic program controlling C-LTMR development and call for a revisit for the physiological functions of C-LTMRs.
doi:10.1523/JNEUROSCI.3942-12.2013
PMCID: PMC3652638  PMID: 23325226
12.  Population coding of somatic sensations 
Neuroscience bulletin  2012;28(2):91-99.
The somatic sensory system includes a variety of sensory modalities, such as touch, pain, itch, and temperature sensitivity. The coding of these modalities appears to be best explained by the population-coding theory, which is composed of the following features. First, an individual somatic sensory afferent is connected with a specific neural circuit or network (for simplicity, a sensory-labeled line), whose isolated activation is sufficient to generate one specific sensation under normal conditions. Second, labeled lines are interconnected through local excitatory and inhibitory interneurons. As a result, activation of one labeled line could modulate, or provide gate control of, another labeled line. Third, most sensory fibers are polymodal, such that a given stimulus placed onto the skin often activates two or multiple sensory-labeled lines; crosstalk among them is needed to generate one dominant sensation. Fourth and under pathological conditions, a disruption of the antagonistic interaction among labeled lines could open normally masked neuronal pathways, and allow a given sensory stimulus to evoke a new sensation, such as pain evoked by innocuous mechanical or thermal stimuli and itch evoked by painful stimuli. As a result of this, some sensory fibers operate along distinct labeled lines under normal versus pathological conditions. Thus, a better understanding of the neural network underlying labeled line crosstalk may provide new strategies to treat chronic pain and itch.
doi:10.1007/s12264-012-1201-2
PMCID: PMC3590490  PMID: 22466120
developmental neurobiology; dorsal root ganglion; pain pathways; itch; spinal dorsal horn
13.  Tlx3 and Runx1 act in combination to coordinate the development of a cohort of nociceptors, thermoceptors and pruriceptors 
The Journal of Neuroscience  2012;32(28):9706-9715.
Neurons in the mouse dorsal root ganglia (DRG) are composed of a variety of sensory modalities, such as pain-related nociceptors, itch-related pruriceptors, and thermoceptors. All these neurons are derived from late-born neurons that are initially marked by the expression of the nerve growth factor receptor TrkA. During perinatal and postnatal development, these TrkA lineage neurons are globally segregated into Ret-expressing and TrkA-expressing subtypes, and start to express a variety of sensory receptors and ion channels. The runt domain transcription factor Runx1 plays a pivotal role in controlling these developmental processes, but it remains unclear how it works. Here we showed that the homeodomain transcription factor Tlx3, expressed broadly in DRG neurons, is required to establish most Runx1-dependent phenotypes, including the segregation of TrkA-expressing versus Ret-expressing neurons and the expression of a dozen of sensory channels and receptors implicated in sensing pain, itch and temperature. Expression of Runx1 and Tlx3 is independent of each other at prenatal stages when they first establish the expression of these channels and receptors. Moreover, overexpression of Runx1 plus Tlx3 was able to induce ectopic expression of sensory channels and receptors. Collectively, these studies suggest that genetically Tlx3 acts in combination with Runx1 to control the development of a cohort of nocicepotors, thermoceptors and pruriceptors in mice.
doi:10.1523/JNEUROSCI.1109-12.2012
PMCID: PMC3405974  PMID: 22787056
14.  TLR3 deficiency impairs spinal cord synaptic transmission, central sensitization, and pruritus in mice 
The Journal of Clinical Investigation  2012;122(6):2195-2207.
Itch, also known as pruritus, is a common, intractable symptom of several skin diseases, such as atopic dermatitis and xerosis. TLRs mediate innate immunity and regulate neuropathic pain, but their roles in pruritus are elusive. Here, we report that scratching behaviors induced by histamine-dependent and -independent pruritogens are markedly reduced in mice lacking the Tlr3 gene. TLR3 is expressed mainly by small-sized primary sensory neurons in dorsal root ganglions (DRGs) that coexpress the itch signaling pathway components transient receptor potential subtype V1 and gastrin-releasing peptide. Notably, we found that treatment with a TLR3 agonist induces inward currents and action potentials in DRG neurons and elicited scratching in WT mice but not Tlr3–/– mice. Furthermore, excitatory synaptic transmission in spinal cord slices and long-term potentiation in the intact spinal cord were impaired in Tlr3–/– mice but not Tlr7–/– mice. Consequently, central sensitization–driven pain hypersensitivity, but not acute pain, was impaired in Tlr3–/– mice. In addition, TLR3 knockdown in DRGs also attenuated pruritus in WT mice. Finally, chronic itch in a dry skin condition was substantially reduced in Tlr3–/– mice. Our findings demonstrate a critical role of TLR3 in regulating sensory neuronal excitability, spinal cord synaptic transmission, and central sensitization. TLR3 may serve as a new target for developing anti-itch treatment.
doi:10.1172/JCI45414
PMCID: PMC3366391  PMID: 22565312
15.  Generation of somatic sensory neuron diversity and implications on sensory coding 
Neurons in the dorsal root ganglia (DRG) are composed of a variety of sensory modalities, three of which are pain-sensing nociceptors, temperature-sensing thermoceptors, and itch-sensing pruriceptors. All these neurons are emerged from a common pool of embryonic DRG neurons that are marked by the expression of the neurotrophin receptor TrkA. Here we discuss how intrinsic transcription factors interface with target-derived signals to specify these functionally distinct sensory neurons. We will also discuss how this control mechanism provides a developmental perspective for the coding of somatic sensations.
doi:10.1016/j.conb.2010.09.003
PMCID: PMC3029488  PMID: 20888752
16.  Genetic marking and characterization of Tac2-expressing neurons in the central and peripheral nervous system 
Molecular Brain  2012;5:3.
Background
The neurocircuits that process somatic sensory information in the dorsal horn of the spinal cord are still poorly understood, with one reason being the lack of Cre lines for genetically marking or manipulating selective subpopulations of dorsal horn neurons. Here we describe Tac2-Cre mice that were generated to express the Cre recombinase gene from the Tac2 locus. Tachykinin 2 (Tac2) encodes a neurotransmitter, neurokinin B (NKB).
Results
By crossing Tac2-Cre mice with ROSA26-tdTomato reporter mice, we directly visualized Tac2 lineage neurons in the dorsal root ganglia, the dorsal horn of the spinal cord, and many parts of the brain including the olfactory bulb, cerebral cortex, amygdala, hippocampus, habenula, hypothalamus, and cerebellum. This Tac2-Cre allele itself was a null allele for the Tac2 gene. Behavioral analyses showed that Tac2 homozygous null mice responded normally to a series of algogenic (pain-inducing) and pruritic (itch-inducing) stimuli.
Conclusions
Tac2-Cre mice are a useful tool to mark specific subsets of neurons in the sensory ganglia, the dorsal spinal cord, and the brain. These mice can also be used for future genetic manipulations to study the functions of Tac2-expressing neurons or the functions of genes expressed in these neurons.
doi:10.1186/1756-6606-5-3
PMCID: PMC3281773  PMID: 22272772
17.  Phenotypic Switching of Nonpeptidergic Cutaneous Sensory Neurons following Peripheral Nerve Injury 
PLoS ONE  2011;6(12):e28908.
In adult mammals, the phenotype of half of all pain-sensing (nociceptive) sensory neurons is tonically modulated by growth factors in the glial cell line-derived neurotrophic factor (GDNF) family that includes GDNF, artemin (ARTN) and neurturin (NRTN). Each family member binds a distinct GFRα family co-receptor, such that GDNF, NRTN and ARTN bind GFRα1, -α2, and -α3, respectively. Previous studies revealed transcriptional regulation of all three receptors in following axotomy, possibly in response to changes in growth factor availability. Here, we examined changes in the expression of GFRα1-3 in response to injury in vivo and in vitro. We found that after dissociation of adult sensory ganglia, up to 27% of neurons die within 4 days (d) in culture and this can be prevented by nerve growth factor (NGF), GDNF and ARTN, but not NRTN. Moreover, up-regulation of ATF3 (a marker of neuronal injury) in vitro could be prevented by NGF and ARTN, but not by GDNF or NRTN. The lack of NRTN efficacy was correlated with rapid and near-complete loss of GFRα2 immunoreactivity. By retrogradely-labeling cutaneous afferents in vivo prior to nerve cut, we demonstrated that GFRα2-positive neurons switch phenotype following injury and begin to express GFRα3 as well as the capsaicin receptor, transient receptor potential vanilloid 1(TRPV1), an important transducer of noxious stimuli. This switch was correlated with down-regulation of Runt-related transcription factor 1 (Runx1), a transcription factor that controls expression of GFRα2 and TRPV1 during development. These studies show that NRTN-responsive neurons are unique with respect to their plasticity and response to injury, and suggest that Runx1 plays an ongoing modulatory role in the adult.
doi:10.1371/journal.pone.0028908
PMCID: PMC3244441  PMID: 22216140
18.  VGLUT2-dependent Glutamate Release from Nociceptors Is Required to Sense Pain and Suppress Itch 
Neuron  2010;68(3):543-556.
SUMMARY
Itch can be suppressed by painful stimuli, but the underlying neural basis is unknown. We generated conditional null mice in which VGLUT2-dependent synaptic glutamate release from mainly Nav1.8-expressing nociceptors was abolished. These mice showed deficits in pain behaviors including mechanical pain, heat pain, capsaicin-evoked pain, inflammatory pain and neuropathic pain. The pain deficits were accompanied by greatly enhanced itching, as suggested by i) sensitization of both histamine-dependent and histamine-independent itch pathways, and ii) development of spontaneous scratching and skin lesions. Strikingly, intradermal capsaicin injection promotes itch responses in these mutant mice, as opposed to pain responses in control littermates. Consequently, co-injection of capsaicin was no longer able to mask itch evoked by pruritogenic compounds. Our studies suggest that synaptic glutamate release from a group of peripheral nociceptors is required to sense pain and suppress itch. Elimination of VGLUT2 in these nociceptors creates a mouse model of chronic neurogenic itch.
doi:10.1016/j.neuron.2010.09.008
PMCID: PMC2991105  PMID: 21040853
19.  Loss of inhibitory interneurons in the dorsal spinal cord and elevated itch in Bhlhb5 mutant mice 
Neuron  2010;65(6):886-898.
SUMMARY
Itch is the least well understood of all the somatic senses, and the neural circuits that underlie this sensation are poorly defined. Here we show that the atonal-related transcription factor Bhlhb5 is transiently expressed in the dorsal horn of the developing spinal cord and appears to play a role in the formation and regulation of pruritic (itch) circuits. Mice lacking Bhlhb5 develop self-inflicted skin lesions and show significantly enhanced scratching responses to pruritic agents. Through genetic fate-mapping and conditional ablation we provide evidence that the pruritic phenotype in Bhlhb5 mutants may be due to selective loss of a subset of inhibitory interneurons in the dorsal horn. Our findings suggest that Bhlhb5 is required for the survival of a specific population of inhibitory interneurons that regulate pruritis and provide evidence that the loss of inhibitory synaptic input results in abnormal itch.
doi:10.1016/j.neuron.2010.02.025
PMCID: PMC2856621  PMID: 20346763
20.  Labeled lines meet and talk: population coding of somatic sensations 
The Journal of Clinical Investigation  2010;120(11):3773-3778.
The somatic sensory system responds to stimuli of distinct modalities, including touch, pain, itch, and temperature sensitivity. In the past century, great progress has been made in understanding the coding of these sensory modalities. From this work, two major features have emerged. First, there are specific neuronal circuits or labeled lines transmitting specific sensory information from the skin to the brain. Second, the generation of specific sensations often involves crosstalk among distinct labeled lines. These features suggest that population coding is the mechanism underlying somatic sensation.
doi:10.1172/JCI43426
PMCID: PMC2964985  PMID: 21041959
21.  RETouching upon Mechanoreceptors 
Neuron  2009;64(6):773-776.
The rapidly adapting (RA) low threshold mechanoreceptors respond to movement of the skin and vibration, and are critical for the perception of texture and shape. In this issue of Neuron, two papers demonstrate that early-born Ret+ sensory neurons are RA mechanoreceptors, whose peripheral nerve terminals are associated with Meissner corpuscles, the longitudinal lanceolate endings, and the Pacinian corpuscles. The studies further show that Ret signaling is essential for the development of these mechanoreceptors.
doi:10.1016/j.neuron.2009.12.014
PMCID: PMC2838379  PMID: 20064382
22.  Characterization of two Runx1-dependent nociceptor differentiation programs necessary for inflammatory versus neuropathic pain 
Molecular Pain  2010;6:45.
Background
The cellular and molecular programs that control specific types of pain are poorly understood. We reported previously that the runt domain transcription factor Runx1 is initially expressed in most nociceptors and controls sensory neuron phenotypes necessary for inflammatory and neuropathic pain.
Results
Here we show that expression of Runx1-dependent ion channels and receptors is distributed into two nociceptor populations that are distinguished by persistent or transient Runx1 expression. Conditional mutation of Runx1 at perinatal stages leads to preferential impairment of Runx1-persistent nociceptors and a selective defect in inflammatory pain. Conversely, constitutive Runx1 expression in Runx1-transient nociceptors leads to an impairment of Runx1-transient nociceptors and a selective deficit in neuropathic pain. Notably, the subdivision of Runx1-persistent and Runx1-transient nociceptors does not follow the classical nociceptor subdivision into IB4+ nonpeptidergic and IB4- peptidergic populations.
Conclusion
Altogether, we have uncovered two distinct Runx1-dependent nociceptor differentiation programs that are permissive for inflammatory versus neuropathic pain. These studies lend support to a transcription factor-based distinction of neuronal classes necessary for inflammatory versus neuropathic pain.
doi:10.1186/1744-8069-6-45
PMCID: PMC2919460  PMID: 20673362
23.  ‘Runxs and regulations’ of sensory and motor neuron subtype differentiation: Implications for hematopoietic development 
Runt-related (RUNX) transcription factors are evolutionarily conserved regulators of a number of developmental mechanisms. RUNX proteins often control the balance between proliferation and differentiation and alterations of their functions are associated with different types of cancer and other human pathologies. Moreover, RUNX factors control important steps during the developmental acquisition of mature phenotypes. A number of investigations are beginning to shed light on the involvement of RUNX family members in the development of the nervous system. This review summarizes recent progress in the study of the roles of mammalian RUNX proteins during the differentiation of sensory and motor neurons in the peripheral and central nervous system, respectively. The implications of those findings for RUNX-mediated regulation of hematopoietic development will also be discussed.
doi:10.1016/j.bcmd.2009.03.001
PMCID: PMC2700053  PMID: 19349198
Acute myeloid leukemia; Differentiation; Dorsal root ganglion; Motor neurons; Nervous system; Proliferation; Runx; Sensory neurons; Spinal cord
24.  A genome-wide screen for spatially restricted expression patterns identifies transcription factors that regulate glial development 
Forward genetic screens in genetically accessible invertebrate organisms such as Drosophila melanogaster have shed light on transcription factors that specify formation of neurons in the vertebrate central nervous system. However, invertebrate models have, to date, been uninformative with respect to genes that specify formation of the vertebrate glial lineages. All recent insights into specification of vertebrate glia have come via monitoring the spatial and temporal expression patterns of individual transcription factors during development. In studies described here, we have taken this approach to the genome scale with an in silico screen of the Mahoney pictorial atlas of transcription factor expression in the developing CNS. From the population of 1445 known or probable transcription factors encoded in the mouse genome we identify 12 novel transcription factors that are expressed in glial lineage progenitor cells. Entry-level screens for biological function establish one of these transcription factors, Klf15, as sufficient for genesis of precocious GFAP positive astrocytes in spinal cord explants. Another transcription factor, Tcf4, plays an important role in maturation of oligodendrocyte progenitors.
doi:10.1523/JNEUROSCI.0160-09.2009
PMCID: PMC2775518  PMID: 19741146
Transcription factor; Glia; Development; Oligodendrocyte; Astrocyte; Spinal Cord
25.  JNK-induced MCP-1 production in spinal cord astrocytes contributes to central sensitization and neuropathic pain 
Our previous study showed that activation of c-jun-N-terminal kinase (JNK) in spinal astrocytes plays an important role in neuropathic pain sensitization. We further investigated how JNK regulates neuropathic pain. In cultured astrocytes, TNF-α transiently activated JNK via TNF receptor-1. Cytokine array indicated that the chemokine CCL2/MCP-1 (monocyte chemoattractant protein-1) was strongly induced by the TNF-α/JNK pathway. MCP-1 upregulation by TNF-α was dose-dependently inhibited by the JNK inhibitors SP600125 and D-JNKI-1. Spinal injection of TNF-α produced JNK-dependent pain hypersensitivity and MCP-1 upregulation in the spinal cord. Further, spinal nerve ligation (SNL) induced persistent neuropathic pain and MCP-1 upregulation in the spinal cord, and both were suppressed by D-JNKI-1. Remarkably, MCP-1 was primarily induced in spinal cord astrocytes after SNL. Spinal administration of MCP-1 neutralizing antibody attenuated neuropathic pain. Conversely, spinal application of MCP-1 induced heat hyperalgesia and phosphorylation of extracellular signal-regulated kinase (ERK) in superficial spinal cord dorsal horn neurons, indicative of central sensitization (hyperactivity of dorsal horn neurons). Patch clamp recordings in lamina II neurons of isolated spinal cord slices showed that MCP-1 not only enhanced spontaneous excitatory synaptic currents (sEPSCs) but also potentiated NMDA- and AMPA-induced currents. Finally, the MCP-1 receptor CCR2 was expressed in neurons and some non-neuronal cells in the spinal cord. Taken together, we have revealed a previously unknown mechanism of MCP-1 induction and action. MCP-1 induction in astrocytes following JNK activation contributes to central sensitization and neuropathic pain facilitation by enhancing excitatory synaptic transmission. Inhibition of the JNK/MCP-1 pathway may provide a new therapy for neuropathic pain management.
doi:10.1523/JNEUROSCI.3623-08.2009
PMCID: PMC2682921  PMID: 19339605
MAP kinase; chemokine; CCL2; CCR2; TNF-α; nerve injury; glia; neural-glial interaction

Results 1-25 (29)