Coexpression of Runx1 and Runx3 in mechanoreceptive dorsal root ganglion neurons

Touch receptor end-organ innervation and function require sensory neuron expression of the transcription factor Meis2

Touch sensation is primarily encoded by mechanoreceptors, called low-threshold mechanoreceptors (LTMRs), with their cell bodies in the dorsal root ganglia. Because of their great diversity in terms of molecular signature, terminal endings morphology, and electrophysiological properties, mirroring the complexity of tactile experience, LTMRs are a model of choice to study the molecular cues differentially controlling neuronal diversification. While the transcriptional codes that define different LTMR subtypes have been extensively studied, the molecular players that participate in their late maturation and in particular in the striking diversity of their end-organ morphological specialization are largely unknown. Here we identified the TALE homeodomain transcription factor Meis2 as a key regulator of LTMRs target-field innervation in mice. Meis2 is specifically expressed in cutaneous LTMRs, and its expression depends on target-derived signals. While LTMRs lacking Meis2 survived and are normally specified, their end-organ innervations, electrophysiological properties, and transcriptome are differentially and markedly affected, resulting in impaired sensory-evoked behavioral responses. These data establish Meis2 as a major transcriptional regulator controlling the orderly formation of sensory neurons innervating peripheral end organs required for light touch.

Calcium-binding protein parvalbumin in the spinal cord and dorsal root ganglia

Parvalbumin is one of the calcium-binding proteins. In the spinal cord, it is mainly expressed in inhibitory neurons; in the dorsal root ganglia, it is expressed in proprioceptive neurons. In contrast to in the brain, weak systematization of parvalbumin-expressing neurons occurs in the spinal cord. The aim of this paper is to provide a systematic review of parvalbumin-expressing neuronal populations throughout the spinal cord and the dorsal root ganglia of mammals, regarding their mapping, co-expression with some functional markers. The data reviewed are mostly concerning rodentia species because they are predominantly presented in literature.

Vertebrate Sensory Ganglia: Common and Divergent Features of the Transcriptional Programs Generating Their Functional Specialization

Sensory fibers of the peripheral nervous system carry sensation from specific sense structures or use different tissues and organs as receptive fields, and convey this information to the central nervous system. In the head of vertebrates, each cranial sensory ganglia and associated nerves perform specific functions. Sensory ganglia are composed of different types of specialized neurons in which two broad categories can be distinguished, somatosensory neurons relaying all sensations that are felt and visceral sensory neurons sensing the internal milieu and controlling body homeostasis. While in the trunk somatosensory neurons composing the dorsal root ganglia are derived exclusively from neural crest cells, somato- and visceral sensory neurons of cranial sensory ganglia have a dual origin, with contributions from both neural crest and placodes. As most studies on sensory neurogenesis have focused on dorsal root ganglia, our understanding of the molecular mechanisms underlying the embryonic development of the different cranial sensory ganglia remains today rudimentary. However, using single-cell RNA sequencing, recent studies have made significant advances in the characterization of the neuronal diversity of most sensory ganglia. Here we summarize the general anatomy, function and neuronal diversity of cranial sensory ganglia. We then provide an overview of our current knowledge of the transcriptional networks controlling neurogenesis and neuronal diversification in the developing sensory system, focusing on cranial sensory ganglia, highlighting specific aspects of their development and comparing it to that of trunk sensory ganglia.

The Role of RUNX1 in NF1-Related Tumors and Blood Disorders

Neurofibromatosis type 1 (NF1) is an autosomal dominant disorder. NF1 patients are predisposed to formation of several type solid tumors as well as to juvenile myelomonocytic leukemia. Loss of NF1 results in dysregulation of MAPK, PI3K and other signaling cascades, to promote cell proliferation and to inhibit cell apoptosis. The RUNX1 gene is associated with stem cell function in many tissues, and plays a key role in the fate of stem cells. Aberrant RUNX1 expression leads to context-dependent tumor development, in which RUNX1 may serve as a tumor suppressor or an oncogene in specific tissue contexts. The co-occurrence of mutation of NF1 and RUNX1 is detected rarely in several cancers and signaling downstream of RAS-MAPK can alter RUNX1 function. Whether aberrant RUNX1 expression contributes to NF1-related tumorigenesis is not fully understood. This review focuses on the role of RUNX1 in NF1-related tumors and blood disorders, and in sporadic cancers.

The emergence of transcriptional identity in somatosensory neurons

More than twelve morphologically and physiologically distinct subtypes of primary somatosensory neuron report salient features of our internal and external environments^1-4. It is unclear how specialized gene expression programs emerge during development to endow these subtypes with their unique properties. To assess the developmental progression of transcriptional maturation of each subtype of principal somatosensory neuron, we generated a transcriptomic atlas of cells traversing the primary somatosensory neuron lineage in mice. Here we show that somatosensory neurogenesis gives rise to neurons in a transcriptionally unspecialized state, characterized by co-expression of transcription factors that become restricted to select subtypes as development proceeds. Single-cell transcriptomic analyses of sensory neurons from mutant mice lacking transcription factors suggest that these broad-to-restricted transcription factors coordinate subtype-specific gene expression programs in subtypes in which their expression is maintained. We also show that neuronal targets are involved in this process; disruption of the prototypic target-derived neurotrophic factor NGF leads to aberrant subtype-restricted patterns of transcription factor expression. Our findings support a model in which cues that emanate from intermediate and final target fields promote neuronal diversification in part by transitioning cells from a transcriptionally unspecialized state to transcriptionally distinct subtypes by modulating the selection of subtype-restricted transcription factors.

RUNX represses Pmp22 to drive neurofibromagenesis

Patients with neurofibromatosis type 1 (NF1) are predisposed to develop neurofibromas, but the underlying molecular mechanisms of neurofibromagenesis are not fully understood. We showed dual genetic deletion of Runx1 and Runx3 in Schwann cells (SCs) and SC precursors delayed neurofibromagenesis and prolonged mouse survival. We identified peripheral myelin protein 22 (Pmp22/Gas3) related to neurofibroma initiation. Knockdown of Pmp22 with short hairpin RNAs increased Runx1^fl/fl;Runx3^fl/fl;Nf1^fl/fl;DhhCre tumor-derived sphere numbers and enabled significantly more neurofibroma-like microlesions on transplantation. Conversely, overexpression of Pmp22 in mouse neurofibroma SCs decreased cell proliferation. Mechanistically, RUNX1/3 regulated alternative promoter usage and induced levels of protein expression of Pmp22 to control SC growth. Last, pharmacological inhibition of RUNX/core-binding factor β (CBFB) activity significantly reduced neurofibroma volume in vivo. Thus, we identified a signaling pathway involving RUNX1/3 suppression of Pmp22 in neurofibroma initiation and/or maintenance. Targeting disruption of RUNX/CBFB interaction might provide a novel therapy for patients with neurofibroma.

The role of Celsr3 in the development of central somatosensory projections from dorsal root ganglia

Dorsal root ganglion (DRG) neurons receive peripheral somatosensory information and send orderly projections to second-order relay nuclei in the spinal cord and in the brainstem. Atypical cadherin Celsr3 is known to play a critical role in wiring of several central and peripheral axons. Although Celsr3 mRNA is heavily expressed in DRG neurons, its role in the development of somatosensory projections remains unexplored. Here we assessed the role of Celsr3 in DRG using conditional gene inactivation in crosses with Wnt1-Cre mice. Using Celsr3-GFP transgenic mice, we found that Celsr3 was highly expressed in different DRG cells, such as Pavalbumin-, TrkB-, and calcitonin gene-related peptide (CGRP)-positive neurons. Wnt1-Cre;Celsr3^f/- animals survived for a few weeks and looked smaller than littermate controls. DiI tracing showed that early DRG axons entered the spinal cord and reached spinal cord targets similarly in mutant and control mice. CGRP-positive fiber density was significantly decreased in lamina I in the mutant versus control spinal cord at postnatal day (P) 7 and P14. Furthermore, more Pavalbumin-positive fibers invaded the gray matter and made more contacts with spinal motor neurons in mutant than in control samples. Behavioral analysis showed that mutant animals were less sensitive to pain and more sensitive to mechanical stimulation than controls. In conclusion, Celsr3 is dispensable for the patterning of central DRG projections, but it regulates for the fine mapping of sensory fibers in the gray matter, which is important for somatosensory processing.

Roles of Runx Genes in Nervous System Development

Runt-related (Runx) transcription factors play essential roles during development and adult tissue homeostasis and are responsible for several human diseases. They regulate a variety of biological mechanisms in numerous cell lineages. Recent years have seen significant progress in our understanding of the functions performed by Runx proteins in the developing and postnatal mammalian nervous system. In both central and peripheral nervous systems, Runx1 and Runx3 display remarkably specific expression in mostly non-overlapping groups of postmitotic neurons. In the central nervous system, Runx1 is involved in the development of selected motor neurons controlling neural circuits mediating vital functions such as chewing, swallowing, breathing, and locomotion. In the peripheral nervous system, Runx1 and Runx3 play essential roles during the development of sensory neurons involved in circuits mediating pain, itch, thermal sensation and sense of relative position. Runx1 and Runx3 orchestrate complex gene expression programs controlling neuronal subtype specification and axonal connectivity. Runx1 is also important in the olfactory system, where it regulates the progenitor-to-neuron transition in undifferentiated neural progenitor cells in the olfactory epithelium as well as the proliferation and developmental maturation of specific glial cells termed olfactory ensheathing cells. Moreover, upregulated Runx expression is associated with brain injury and disease. Increasing knowledge of the functions of Runx proteins in the developing and postnatal nervous system is therefore expected to improve our understanding of nervous system development, homeostasis and disease.

The specification and wiring of mammalian cutaneous low-threshold mechanoreceptors

The mammalian cutaneous low-threshold mechanoreceptors (LTMRs) are a diverse set of primary somatosensory neurons that function to sense external mechanical force. Generally, LTMRs are composed of Aβ-LTMRs, Aδ-LTMRs, and C-LTMRs, which have distinct molecular, physiological, anatomical, and functional features. The specification and wiring of each type of mammalian cutaneous LTMRs is established during development by the interplay of transcription factors with trophic factor signalling. In this review, we summarize the cohort of extrinsic and intrinsic factors generating the complex mammalian cutaneous LTMR circuits that mediate our tactile sensations and behaviors. For further resources related to this article, please visit the WIREs website.

Restriction of Nonpermissive RUNX3 Protein Expression in T Lymphocytes by the Kozak Sequence

The transcription factor Runx3 promotes differentiation of naive CD4(+) T cells into type-1 effector T (TH1) cells at the expense of TH2. TH1 cells as well as CD8(+) T cells express a subset-specific Runx3 transcript from a distal promoter, which is necessary for high protein expression. However, all T cell subsets, including naive CD4(+) T cells and TH2 cells, express a distinct transcript of Runx3 that is derived from a proximal promoter and that produces functional protein in neurons. Therefore, accumulation of RUNX3 protein generated from the proximal transcript needs to be repressed at the posttranscriptional level to preserve CD4(+) T cell capability of differentiating into TH2 cells. In this article, we show that expression of RUNX3 protein from the proximal Runx3 transcript is blocked at the level of translational initiation in T cells. A coding sequence for the proximal Runx3 mRNA is preceded by a nonoptimal context sequence for translational initiation, known as the Kozak sequence, and thus generates protein at low efficiencies and with multiple alternative translational initiations. Editing the endogenous initiation context to an "optimal" Kozak sequence in a human T cell line resulted in enhanced translation of a single RUNX3 protein derived from the proximal transcript. Furthermore, RUNX3 protein represses transcription from the proximal promoter in T cells. These results suggest that nonpermissive expression of RUNX3 protein is restricted at the translational level, and that the repression is further enforced by a transcriptional regulation for maintenance of diverse developmental plasticity of T cells for different effector subsets.

Runx3-regulated expression of two Ntrk3 transcript variants in dorsal root ganglion neurons

Somatosensation is divided into proprioception and cutaneous sensation. Dorsal root ganglion (DRG) neurons project their fibers toward peripheral targets including muscles and skin, and centrally to the spinal cord. Proprioceptive DRG neurons transmit information from muscle spindles and Golgi tendon organs to the spinal cord. We previously showed that Runt-related transcription factor 3 (Runx3) is expressed in these neurons and their projections to the ventral spinal cord and muscle spindles are lost in Runx3-deficient (Runx3(-/-) ) mouse embryos. Although Runx3 is likely to contribute to the fate decision and projection of proprioceptive DRG neurons, the precise roles for Runx3 in these phenomena are unknown. To identify genes regulated by Runx3 in embryonic DRGs, we performed microarray analyses using cDNAs isolated from wild-type and Runx3(-/-) DRGs of embryonic day (E) 12.5 and selected two transcript variants of the tyrosine kinase receptor C (TrkC) gene. These variants, Ntrk3 variant 1 (Ntrk3-v1) and variant 2 (Ntrk3-v2), encode full-length and truncated receptors of neurotrophin-3, respectively. Using double in situ hybridization, we found that most of Ntrk3-v1 mRNA expression in E14.5 DRGs depended on Runx3 but that more than half of Ntrk3-v2 mRNA one were expressed in a Runx3-independent manner. Furthermore, our data revealed that the rate of Ntrk3-v1 and Ntrk3-v2 colocalization in DRGs changed from E14.5 to E18.5. Together, our data suggest that Runx3 may play a crucial role in the development of DRGs by regulating the expression of Ntrk3 variants and that DRG neurons expressing Ntrk3-v1 but not Ntrk3-v2 may differentiate into proprioceptive ones.

Contribution of the Runx1 transcription factor to axonal pathfinding and muscle innervation by hypoglossal motoneurons

The runt-related transcription factor Runx1 contributes to cell type specification and axonal targeting projections of the nociceptive dorsal root ganglion neurons. Runx1 is also expressed in the central nervous system, but little is known of its functions in brain development. At mouse embryonic day (E) 17.5, Runx1-positive neurons were detected in the ventrocaudal subdivision of the hypoglossal nucleus. Runx1-positive neurons lacked calcitonin gene-related peptide (CGRP) expression, whereas Runx1-negative neurons expressed CGRP. Expression of CGRP was not changed in Runx1-deficient mice at E17.5, suggesting that Runx1 alone does not suppress CGRP expression. Hypoglossal axon projections to the intrinsic vertical (V) and transverse (T) tongue muscles were sparser in Runx1-deficient mice at E17.5 compared to age-matched wild-type littermates. Concomitantly, vesicular acetylcholine transporter-positive axon terminals and acetylcholine receptor clusters were less dense in the V and T tongue muscles of Runx1-deficient mice. These abnormalities in axonal projection were not caused by a reduction in the total number hypoglossal neurons, failed synaptogenesis, or tongue muscles deficits. Our results implicate Runx1 in the targeting of ventrocaudal hypoglossal axons to specific tongue muscles. However, Runx1 deficiency did not alter neuronal survival or the expression of multiple motoneuron markers as in other neuronal populations. Thus, Runx1 appears to have distinct developmental functions in different brain regions.

Neurotrophin signalling and transcription programmes interactions in the development of somatosensory neurons

Somatosensory neurons of the dorsal root ganglia are generated from multipotent neural crest cells by a process of progressive specification and differentiation. Intrinsic transcription programmes active in somatosensory neuron progenitors and early post-mitotic neurons drive the cell-type expression of neurotrophin receptors. In turn, signalling by members of the neurotrophin family controls expression of transcription factors that regulate neuronal sub-type specification. This chapter explores the mechanisms by which this crosstalk between neurotrophin signalling and transcription programmes generates the diverse functional sub-types of somatosensory neurons found in the mature animal.

The anatomy, function, and development of mammalian Aβ low-threshold mechanoreceptors

Touch sensation is critical for our social and environmental interactions. In mammals, most discriminative light touch sensation is mediated by the Aβ low-threshold mechanoreceptors. Cell bodies of Aβ low-threshold mechanoreceptors are located in the dorsal root ganglia and trigeminal ganglia, which extend a central projection innervating the spinal cord and brain stem and a peripheral projection innervating the specialized mechanosensory end organs. These specialized mechanosensory end organs include Meissner's corpuscles, Pacinian corpuscles, lanceolate endings, Merkel cells, and Ruffini corpuscles. The morphologies and physiological properties of these mechanosensory end organs and their innervating neurons have been investigated for over a century. In addition, recent advances in mouse genetics have enabled the identification of molecular mechanisms underlying the development of Aβ low-threshold mechanoreceptors, which highlight the crucial roles of neurotrophic factor signaling and transcription factor activity in this process. Here, we will review the anatomy, physiological properties, and development of mammalian low-threshold Aβ mechanoreceptors.