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Homma S, Shimada T, Wada I, Kumaki K, Sato N, Yaginuma H. A three-component model of the spinal nerve ramification: Bringing together the human gross anatomy and modern Embryology. Front Neurosci 2023; 16:1009542. [PMID: 36726852 PMCID: PMC9884977 DOI: 10.3389/fnins.2022.1009542] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/02/2022] [Accepted: 12/05/2022] [Indexed: 01/17/2023] Open
Abstract
Due to its long history, the study of human gross anatomy has not adequately incorporated modern embryological findings; consequently, the current understanding has often been incompatible with recent discoveries from molecular studies. Notably, the traditional epaxial and hypaxial muscle distinction, and their corresponding innervation by the dorsal and ventral rami of the spinal nerve, do not correspond to the primaxial and abaxial muscle distinction, defined by the mesodermal lineages of target tissues. To resolve the disagreement between adult anatomy and embryology, we here propose a novel hypothetical model of spinal nerve ramification. Our model is based on the previously unknown developmental process of the intercostal nerves. Observations of these nerves in the mouse embryos revealed that the intercostal nerves initially had superficial and deep ventral branches, which is contrary to the general perception of a single ventral branch. The initial dual innervation pattern later changes into an adult-like single branch pattern following the retraction of the superficial branch. The modified intercostal nerves consist of the canonical ventral branches and novel branches that run on the muscular surface of the thorax, which sprout from the lateral cutaneous branches. We formulated the embryonic branching pattern into the hypothetical ramification model of the human spinal nerve so that the branching pattern is compatible with the developmental context of the target muscles. In our model, every spinal nerve consists of three components: (1) segmental branches that innervate the primaxial muscles, including the dorsal rami, and short branches and long superficial anterior branches from the ventral rami; (2) plexus-forming intramural branches, the serial homolog of the canonical intercostal nerves, which innervate the abaxial portion of the body wall; and (3) plexus-forming extramural branches, the series of novel branches located outside of the body wall, which innervate the girdle and limb muscles. The selective elaboration or deletion of each component successfully explains the reasoning for the standard morphology and variability of the spinal nerve. Therefore, our model brings a novel understanding of spinal nerve development and valuable information for basic and clinical sciences regarding the diverse branching patterns of the spinal nerve.
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Affiliation(s)
- Shunsaku Homma
- Department of Neuroanatomy and Embryology, Fukushima Medical University, Fukushima, Japan
| | - Takako Shimada
- Department of Neuroanatomy and Embryology, Fukushima Medical University, Fukushima, Japan
| | - Ikuo Wada
- Department of Cell Science, Institute of Biomedical Sciences, Fukushima Medical University, Fukushima, Japan
| | - Katsuji Kumaki
- Division of Gross Anatomy and Morphogenesis, Graduate School of Medical and Dental Sciences, Niigata University, Niigata, Japan
| | - Noboru Sato
- Division of Gross Anatomy and Morphogenesis, Graduate School of Medical and Dental Sciences, Niigata University, Niigata, Japan
| | - Hiroyuki Yaginuma
- Department of Neuroanatomy and Embryology, Fukushima Medical University, Fukushima, Japan
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Single-cell transcriptomic analysis reveals diversity within mammalian spinal motor neurons. Nat Commun 2023; 14:46. [PMID: 36596814 PMCID: PMC9810664 DOI: 10.1038/s41467-022-35574-x] [Citation(s) in RCA: 9] [Impact Index Per Article: 9.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/25/2022] [Accepted: 12/12/2022] [Indexed: 01/05/2023] Open
Abstract
Spinal motor neurons (MNs) integrate sensory stimuli and brain commands to generate movements. In vertebrates, the molecular identities of the cardinal MN types such as those innervating limb versus trunk muscles are well elucidated. Yet the identities of finer subtypes within these cell populations that innervate individual muscle groups remain enigmatic. Here we investigate heterogeneity in mouse MNs using single-cell transcriptomics. Among limb-innervating MNs, we reveal a diverse neuropeptide code for delineating putative motor pool identities. Additionally, we uncover that axial MNs are subdivided into three molecularly distinct subtypes, defined by mediolaterally-biased Satb2, Nr2f2 or Bcl11b expression patterns with different axon guidance signatures. These three subtypes are present in chicken and human embryos, suggesting a conserved axial MN expression pattern across higher vertebrates. Overall, our study provides a molecular resource of spinal MN types and paves the way towards deciphering how neuronal subtypes evolved to accommodate vertebrate motor behaviors.
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Dasen JS. Establishing the Molecular and Functional Diversity of Spinal Motoneurons. ADVANCES IN NEUROBIOLOGY 2022; 28:3-44. [PMID: 36066819 DOI: 10.1007/978-3-031-07167-6_1] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/15/2023]
Abstract
Spinal motoneurons are a remarkably diverse class of neurons responsible for facilitating a broad range of motor behaviors and autonomic functions. Studies of motoneuron differentiation have provided fundamental insights into the developmental mechanisms of neuronal diversification, and have illuminated principles of neural fate specification that operate throughout the central nervous system. Because of their relative anatomical simplicity and accessibility, motoneurons have provided a tractable model system to address multiple facets of neural development, including early patterning, neuronal migration, axon guidance, and synaptic specificity. Beyond their roles in providing direct communication between central circuits and muscle, recent studies have revealed that motoneuron subtype-specific programs also play important roles in determining the central connectivity and function of motor circuits. Cross-species comparative analyses have provided novel insights into how evolutionary changes in subtype specification programs may have contributed to adaptive changes in locomotor behaviors. This chapter focusses on the gene regulatory networks governing spinal motoneuron specification, and how studies of spinal motoneurons have informed our understanding of the basic mechanisms of neuronal specification and spinal circuit assembly.
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Affiliation(s)
- Jeremy S Dasen
- NYU Neuroscience Institute, Department of Neuroscience and Physiology, NYU School of Medicine, New York, NY, USA.
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Nagashima H, Koga D, Kusumi S, Mukaigasa K, Yaginuma H, Ushiki T, Sato N. Novel concept for the epaxial/hypaxial boundary based on neuronal development. J Anat 2020; 237:427-438. [PMID: 32786168 DOI: 10.1111/joa.13219] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/30/2019] [Revised: 04/23/2020] [Accepted: 04/23/2020] [Indexed: 11/30/2022] Open
Abstract
Trunk muscles in vertebrates are classified as either dorsal epaxial or ventral hypaxial muscles. Epaxial and hypaxial muscles are defined as muscles innervated by the dorsal and ventral rami of spinal nerves, respectively. Each cluster of spinal motor neurons passing through dorsal rami innervates epaxial muscles, whereas clusters traveling on the ventral rami innervate hypaxial muscles. Herein, we show that some motor neurons exhibiting molecular profiles for epaxial muscles follow a path in the ventral rami. Dorsal deep-shoulder muscles and some body wall muscles are defined as hypaxial due to innervation via the ventral rami, but a part of these ventral rami has the molecular profile of motor neurons that innervate epaxial muscles. Thus, the epaxial and hypaxial boundary cannot be determined simply by the ramification pattern of spinal nerves. We propose that, although muscle innervation occurs via the ventral rami, dorsal deep-shoulder muscles and some body wall muscles represent an intermediate group that lies between epaxial and hypaxial muscles.
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Affiliation(s)
- Hiroshi Nagashima
- Division of Gross Anatomy and Morphogenesis, Niigata University Graduate School of Medical and Dental Sciences, Niigata, Japan
| | - Daisuke Koga
- Division of Microscopic Anatomy and Bio-imaging, Niigata University Graduate School of Medical and Dental Sciences, Niigata, Japan
| | - Satoshi Kusumi
- Division of Microscopic Anatomy and Bio-imaging, Niigata University Graduate School of Medical and Dental Sciences, Niigata, Japan
| | - Katsuki Mukaigasa
- Department of Neuroanatomy and Embryology, Fukushima Medical University, Fukushima, Japan
| | - Hiroyuki Yaginuma
- Department of Neuroanatomy and Embryology, Fukushima Medical University, Fukushima, Japan
| | - Tatsuo Ushiki
- Division of Microscopic Anatomy and Bio-imaging, Niigata University Graduate School of Medical and Dental Sciences, Niigata, Japan
| | - Noboru Sato
- Division of Gross Anatomy and Morphogenesis, Niigata University Graduate School of Medical and Dental Sciences, Niigata, Japan
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Koizumi M. Two mammalian species in which the intercostal nerves innervate the serratus anterior or scalenus muscles together with the cervical nerves: an important clue to clarify the homology of cervico-thoracic trunk muscles in mammals. Anat Sci Int 2019; 94:295-306. [DOI: 10.1007/s12565-019-00487-1] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/04/2018] [Accepted: 04/12/2019] [Indexed: 11/28/2022]
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D'Elia KP, Dasen JS. Development, functional organization, and evolution of vertebrate axial motor circuits. Neural Dev 2018; 13:10. [PMID: 29855378 PMCID: PMC5984435 DOI: 10.1186/s13064-018-0108-7] [Citation(s) in RCA: 18] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/14/2018] [Accepted: 04/26/2018] [Indexed: 12/20/2022] Open
Abstract
Neuronal control of muscles associated with the central body axis is an ancient and essential function of the nervous systems of most animal species. Throughout the course of vertebrate evolution, motor circuits dedicated to control of axial muscle have undergone significant changes in their roles within the motor system. In most fish species, axial circuits are critical for coordinating muscle activation sequences essential for locomotion and play important roles in postural correction. In tetrapods, axial circuits have evolved unique functions essential to terrestrial life, including maintaining spinal alignment and breathing. Despite the diverse roles of axial neural circuits in motor behaviors, the genetic programs underlying their assembly are poorly understood. In this review, we describe recent studies that have shed light on the development of axial motor circuits and compare and contrast the strategies used to wire these neural networks in aquatic and terrestrial vertebrate species.
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Affiliation(s)
- Kristen P D'Elia
- Neuroscience Institute, Department of Neuroscience and Physiology, NYU School of Medicine, New York, NY, 10016, USA
| | - Jeremy S Dasen
- Neuroscience Institute, Department of Neuroscience and Physiology, NYU School of Medicine, New York, NY, 10016, USA.
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Luxey M, Laussu J, Davy A. EphrinB2 sharpens lateral motor column division in the developing spinal cord. Neural Dev 2015; 10:25. [PMID: 26503288 PMCID: PMC4624581 DOI: 10.1186/s13064-015-0051-9] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/18/2015] [Accepted: 10/13/2015] [Indexed: 12/25/2022] Open
Abstract
Background During sensori-motor circuit development, the somas of motoneurons (MN) are distributed in a topographic manner in the ventral horn of the neural tube. Indeed, their position within the lateral motor columns (LMC) correlates with axonal trajectories and identity of target limb muscles. The mechanisms by which this topographic distribution is established remains poorly understood. To address this issue, we assessed the role of ephrinB2 in MN topographic organization in the developing mouse spinal cord. Results First, we used a reporter mouse line to establish the spatio-temporal expression pattern of EfnB2 in the developing LMC. We show that early in LMC development, ephrinB2 is differentially expressed in MN of the lateral versus medial LMC, suggesting a possible role in MN sorting and/or migration. We demonstrate that while MN-specific excision of EfnB2 did not perturb specification or migration of MN, conditional loss of ephrinB2 led to the blurring of the LMC divisional boundary and to errors in the selection of LMC axon trajectory in the limb. Conclusions Altogether, our study uncovered a novel cell autonomous role for ephrinB2 in LMC MN thus emphasizing the prevalent role of this ephrin member in maintaining cell population boundaries. Electronic supplementary material The online version of this article (doi:10.1186/s13064-015-0051-9) contains supplementary material, which is available to authorized users.
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Affiliation(s)
- Maëva Luxey
- Centre de Biologie du Développement, CNRS, 118 Route de Narbonne, 31062, Toulouse, France.,Université de Toulouse, Toulouse, France.,Institut de Recherche Clinique de Montréal, 110 avenue des Pins Ouest, Montréal (Québec), H2W 1R7, Canada
| | - Julien Laussu
- Centre de Biologie du Développement, CNRS, 118 Route de Narbonne, 31062, Toulouse, France.,Université de Toulouse, Toulouse, France
| | - Alice Davy
- Centre de Biologie du Développement, CNRS, 118 Route de Narbonne, 31062, Toulouse, France. .,Université de Toulouse, Toulouse, France.
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Goetz C, Pivetta C, Arber S. Distinct limb and trunk premotor circuits establish laterality in the spinal cord. Neuron 2014; 85:131-144. [PMID: 25543457 DOI: 10.1016/j.neuron.2014.11.024] [Citation(s) in RCA: 35] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 11/24/2014] [Indexed: 12/31/2022]
Abstract
Movement coordination between opposite body sides relies on neuronal circuits capable of controlling muscle contractions according to motor commands. Trunk and limb muscles engage in distinctly lateralized behaviors, yet how regulatory spinal circuitry differs is less clear. Here, we intersect virus technology and mouse genetics to unravel striking distribution differences of interneurons connected to functionally distinct motor neurons. We find that premotor interneurons conveying information to axial motor neurons reside in symmetrically balanced locations while mostly ipsilateral premotor interneurons synapse with limb-innervating motor neurons, especially those innervating more distal muscles. We show that observed distribution differences reflect specific premotor interneuron subpopulations defined by genetic and neurotransmitter identity. Synaptic input across the midline reaches axial motor neurons preferentially through commissural axon arborization, and to a lesser extent, through midline-crossing dendrites capturing contralateral synaptic input. Together, our findings provide insight into principles of circuit organization underlying weighted lateralization of movement.
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Affiliation(s)
- Cyrill Goetz
- Biozentrum, Department of Cell Biology, University of Basel, Klingelbergstrasse 70, 4056 Basel, Switzerland; Friedrich Miescher Institute for Biomedical Research, Maulbeerstrasse 66, 4058 Basel, Switzerland
| | - Chiara Pivetta
- Biozentrum, Department of Cell Biology, University of Basel, Klingelbergstrasse 70, 4056 Basel, Switzerland; Friedrich Miescher Institute for Biomedical Research, Maulbeerstrasse 66, 4058 Basel, Switzerland
| | - Silvia Arber
- Biozentrum, Department of Cell Biology, University of Basel, Klingelbergstrasse 70, 4056 Basel, Switzerland; Friedrich Miescher Institute for Biomedical Research, Maulbeerstrasse 66, 4058 Basel, Switzerland.
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Stifani N. Motor neurons and the generation of spinal motor neuron diversity. Front Cell Neurosci 2014; 8:293. [PMID: 25346659 PMCID: PMC4191298 DOI: 10.3389/fncel.2014.00293] [Citation(s) in RCA: 178] [Impact Index Per Article: 17.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/31/2013] [Accepted: 09/02/2014] [Indexed: 11/13/2022] Open
Abstract
Motor neurons (MNs) are neuronal cells located in the central nervous system (CNS) controlling a variety of downstream targets. This function infers the existence of MN subtypes matching the identity of the targets they innervate. To illustrate the mechanism involved in the generation of cellular diversity and the acquisition of specific identity, this review will focus on spinal MNs (SpMNs) that have been the core of significant work and discoveries during the last decades. SpMNs are responsible for the contraction of effector muscles in the periphery. Humans possess more than 500 different skeletal muscles capable to work in a precise time and space coordination to generate complex movements such as walking or grasping. To ensure such refined coordination, SpMNs must retain the identity of the muscle they innervate. Within the last two decades, scientists around the world have produced considerable efforts to elucidate several critical steps of SpMNs differentiation. During development, SpMNs emerge from dividing progenitor cells located in the medial portion of the ventral neural tube. MN identities are established by patterning cues working in cooperation with intrinsic sets of transcription factors. As the embryo develop, MNs further differentiate in a stepwise manner to form compact anatomical groups termed pools connecting to a unique muscle target. MN pools are not homogeneous and comprise subtypes according to the muscle fibers they innervate. This article aims to provide a global view of MN classification as well as an up-to-date review of the molecular mechanisms involved in the generation of SpMN diversity. Remaining conundrums will be discussed since a complete understanding of those mechanisms constitutes the foundation required for the elaboration of prospective MN regeneration therapies.
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Affiliation(s)
- Nicolas Stifani
- Medical Neuroscience, Dalhousie University Halifax, NS, Canada
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10
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Allodi I, Hedlund E. Directed midbrain and spinal cord neurogenesis from pluripotent stem cells to model development and disease in a dish. Front Neurosci 2014; 8:109. [PMID: 24904255 PMCID: PMC4033221 DOI: 10.3389/fnins.2014.00109] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/03/2014] [Accepted: 04/28/2014] [Indexed: 12/29/2022] Open
Abstract
Induction of specific neuronal fates is restricted in time and space in the developing CNS through integration of extrinsic morphogen signals and intrinsic determinants. Morphogens impose regional characteristics on neural progenitors and establish distinct progenitor domains. Such domains are defined by unique expression patterns of fate determining transcription factors. These processes of neuronal fate specification can be recapitulated in vitro using pluripotent stem cells. In this review, we focus on the generation of dopamine neurons and motor neurons, which are induced at ventral positions of the neural tube through Sonic hedgehog (Shh) signaling, and defined at anteroposterior positions by fibroblast growth factor (Fgf) 8, Wnt1, and retinoic acid (RA). In vitro utilization of these morphogenic signals typically results in the generation of multiple neuronal cell types, which are defined at the intersection of these signals. If the purpose of in vitro neurogenesis is to generate one cell type only, further lineage restriction can be accomplished by forced expression of specific transcription factors in a permissive environment. Alternatively, cell-sorting strategies allow for selection of neuronal progenitors or mature neurons. However, modeling development, disease and prospective therapies in a dish could benefit from structured heterogeneity, where desired neurons are appropriately synaptically connected and thus better reflect the three-dimensional structure of that region. By modulating the extrinsic environment to direct sequential generation of neural progenitors within a domain, followed by self-organization and synaptic establishment, a reductionist model of that brain region could be created. Here we review recent advances in neuronal fate induction in vitro, with a focus on the interplay between cell intrinsic and extrinsic factors, and discuss the implications for studying development and disease in a dish.
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Affiliation(s)
- Ilary Allodi
- Department of Neuroscience, Karolinska Institutet Stockholm, Sweden
| | - Eva Hedlund
- Department of Neuroscience, Karolinska Institutet Stockholm, Sweden
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Kania A. Spinal motor neuron migration and the significance of topographic organization in the nervous system. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2014; 800:133-48. [PMID: 24243104 DOI: 10.1007/978-94-007-7687-6_8] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/19/2022]
Abstract
The nervous system displays a high degree of topographic organisation such that neuronal soma position is closely correlated to axonal trajectory. One example of such order is the myotopic organisation of the motor system where spinal motor neuron position parallels that of target muscles. This chapter will discuss the molecular mechanisms underlying motor neuron soma positioning, which include transcriptional control of Reelin signaling and cadherin expression. As the same transcription factors have been shown to control motor axon innervation of target muscles, a simple mechanism of topographic organisation specification is becoming evident raising the question of how coordinating soma position with axon trajectory might be important for nervous system wiring and its function.
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Affiliation(s)
- Artur Kania
- Institut de recherches cliniques de Montréal (IRCM), 110, ave. des Pins Ouest, Montréal, QC, H2W 1R7, Canada,
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12
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MicroRNA-9 promotes the switch from early-born to late-born motor neuron populations by regulating Onecut transcription factor expression. Dev Biol 2013; 386:358-70. [PMID: 24374159 DOI: 10.1016/j.ydbio.2013.12.023] [Citation(s) in RCA: 29] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/02/2013] [Revised: 12/11/2013] [Accepted: 12/13/2013] [Indexed: 11/23/2022]
Abstract
Motor neurons in the vertebrate spinal cord are stereotypically organized along the rostro-caudal axis in discrete columns that specifically innervate peripheral muscle domains. Originating from the same progenitor domain, the generation of spinal motor neurons is orchestrated by a spatially and temporally tightly regulated set of secreted molecules and transcription factors such as retinoic acid and the Lim homeodomain transcription factors Isl1 and Lhx1. However, the molecular interactions between these factors remained unclear. In this study we examined the role of the microRNA 9 (miR-9) in the specification of spinal motor neurons and identified Onecut1 (OC1) as one of its targets. miR-9 and OC1 are expressed in mutually exclusive patterns in the developing chick spinal cord, with high OC1 levels in early-born motor neurons and high miR-9 levels in late-born motor neurons. miR-9 efficiently represses OC1 expression in vitro and in vivo. Overexpression of miR-9 leads to an increase in late-born neurons, while miR-9 loss-of-function induces additional OC1(+) motor neurons that display a transcriptional profile typical of early-born neurons. These results demonstrate that regulation of OC1 by miR-9 is a crucial step in the specification of spinal motor neurons and support a model in which miR-9 expression in late-born LMCl neurons downregulates Isl1 expression through inhibition of OC1. In conclusion, our study contributes essential factors to the molecular network specifying spinal motor neurons and emphasizes the importance of microRNAs as key players in the generation of neuronal diversity.
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13
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Ricard MJ, Gudas LJ. Cytochrome p450 cyp26a1 alters spinal motor neuron subtype identity in differentiating embryonic stem cells. J Biol Chem 2013; 288:28801-13. [PMID: 23946489 DOI: 10.1074/jbc.m113.474254] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/13/2023] Open
Abstract
The ability to differentiate embryonic stem cells (ESCs) into specific cell types is critical for improved regenerative medicine strategies, cancer chemotherapeutic approaches, and regimens to combat chronic diseases associated with aging. Subclasses of motor neurons (MNs) are generated at different positions along the rostrocaudal axis of the spinal cord, and the signals that specify MN subtype fates remain poorly defined. We show here that the cytochrome P450 enzyme Cyp26a1, which metabolizes all-trans-retinoic acid (RA) and thereby reduces RA levels, plays a crucial role in specifying MN columnar subtypes. Lack of Cyp26a1 in ESCs during differentiation to spinal MNs increases Aldh1a2 (RALDH2) and Hoxc6, markers of the Hox-dependent, lateral motor column (LMC) subtype identity. In contrast, Lhx3, a marker for median motor column identity, showed lower expression in Cyp26a1(-/-)-derived MNs compared with WT. Without Cyp26a1, an increase in intracellular RA concentration plus sonic hedgehog agonist treatment confer an LMC fate on differentiating MNs. Our data suggest a strategy for increasing LMC-type MNs from ESCs by blocking Cyp26a1 in cell replacement/ESC differentiation therapy to treat neurodegenerative diseases, such as amyotrophic lateral sclerosis.
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14
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Lacombe J, Hanley O, Jung H, Philippidou P, Surmeli G, Grinstein J, Dasen JS. Genetic and functional modularity of Hox activities in the specification of limb-innervating motor neurons. PLoS Genet 2013; 9:e1003184. [PMID: 23359544 PMCID: PMC3554521 DOI: 10.1371/journal.pgen.1003184] [Citation(s) in RCA: 60] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/09/2012] [Accepted: 11/06/2012] [Indexed: 11/18/2022] Open
Abstract
A critical step in the assembly of the neural circuits that control tetrapod locomotion is the specification of the lateral motor column (LMC), a diverse motor neuron population targeting limb musculature. Hox6 paralog group genes have been implicated as key determinants of LMC fate at forelimb levels of the spinal cord, through their ability to promote expression of the LMC-restricted genes Foxp1 and Raldh2 and to suppress thoracic fates through exclusion of Hoxc9. The specific roles and mechanisms of Hox6 gene function in LMC neurons, however, are not known. We show that Hox6 genes are critical for diverse facets of LMC identity and define motifs required for their in vivo specificities. Although Hox6 genes are necessary for generating the appropriate number of LMC neurons, they are not absolutely required for the induction of forelimb LMC molecular determinants. In the absence of Hox6 activity, LMC identity appears to be preserved through a diverse array of Hox5–Hox8 paralogs, which are sufficient to reprogram thoracic motor neurons to an LMC fate. In contrast to the apparently permissive Hox inputs to early LMC gene programs, individual Hox genes, such as Hoxc6, have specific roles in promoting motor neuron pool diversity within the LMC. Dissection of motifs required for Hox in vivo specificities reveals that either cross-repressive interactions or cooperativity with Pbx cofactors are sufficient to induce LMC identity, with the N-terminus capable of promoting columnar, but not pool, identity when transferred to a heterologous homeodomain. These results indicate that Hox proteins orchestrate diverse aspects of cell fate specification through both the convergent regulation of gene programs regulated by many paralogs and also more restricted actions encoded through specificity determinants in the N-terminus. Coordinated motor behaviors—as complex as playing a musical instrument or as simple as walking—rely on the ability of motor neurons within the spinal cord to navigate towards and establish specific connections with muscles in the limbs. The establishment of connections between motor neurons and limb muscles is mediated through the actions of genes encoding Hox proteins, a large family of transcription factors conserved amongst all metazoans. However, the specific requirements for Hox genes in motor neuron specification and patterns of muscle connectivity are poorly understood. We have found that members of the Hox6 gene paralog group (Hoxa6, Hoxc6, and Hoxb6) contribute to diverse aspects of motor neuron subtype differentiation. Hox6 gene activity is required during two critical phases of motor neuron development: first as motor axons select a trajectory toward the forelimb and second as they choose specific muscles to innervate. At the molecular level, these two functions are encoded by distinct peptide domains within Hox proteins. This work indicates that Hox proteins execute their critical functions in motor neurons through intrinsic modules that confer distinct specificities and that these activities are central in the genetic network required for motor neuron differentiation.
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Affiliation(s)
- Julie Lacombe
- Smilow Neuroscience Program, Department of Physiology and Neuroscience, Howard Hughes Medical Institute, NYU School of Medicine, New York, New York, United States of America
| | - Olivia Hanley
- Smilow Neuroscience Program, Department of Physiology and Neuroscience, Howard Hughes Medical Institute, NYU School of Medicine, New York, New York, United States of America
| | - Heekyung Jung
- Smilow Neuroscience Program, Department of Physiology and Neuroscience, Howard Hughes Medical Institute, NYU School of Medicine, New York, New York, United States of America
| | - Polyxeni Philippidou
- Smilow Neuroscience Program, Department of Physiology and Neuroscience, Howard Hughes Medical Institute, NYU School of Medicine, New York, New York, United States of America
| | - Gulsen Surmeli
- Department of Biochemistry and Molecular Biophysics, Columbia University, New York, New York, United States of America
| | - Jonathan Grinstein
- Smilow Neuroscience Program, Department of Physiology and Neuroscience, Howard Hughes Medical Institute, NYU School of Medicine, New York, New York, United States of America
| | - Jeremy S. Dasen
- Smilow Neuroscience Program, Department of Physiology and Neuroscience, Howard Hughes Medical Institute, NYU School of Medicine, New York, New York, United States of America
- * E-mail:
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15
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Tschopp P, Christen AJ, Duboule D. Bimodal control of Hoxd gene transcription in the spinal cord defines two regulatory subclusters. Development 2012; 139:929-39. [PMID: 22278926 DOI: 10.1242/dev.076794] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022]
Abstract
The importance of Hox genes in the specification of neuronal fates in the spinal cord has long been recognized. However, the transcriptional controls underlying their collinear expression domains remain largely unknown. Here we show in mice that the correspondence between the physical order of Hoxd genes and their rostral expression boundaries, although respecting spatial collinearity, does not display a fully progressive distribution. Instead, two major anteroposterior boundaries are detected, coinciding with the functional subdivision of the spinal cord. Tiling array analyses reveal two distinct blocks of transcription, regulated independently from one another, that define the observed expression boundaries. Targeted deletions in vivo that remove the genomic fragments separating the two blocks induce ectopic expression of posterior genes. We further evaluate the independent regulatory potential and transcription profile of each gene locus by a tiling array approach using a contiguous series of transgenes combined with locus-specific deletions. Our work uncovers a bimodal type of HoxD spatial collinearity in the developing spinal cord that relies on two separate 'enhancer mini-hubs' to ensure correct Hoxd gene expression levels while maintaining their appropriate anteroposterior boundaries.
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Affiliation(s)
- Patrick Tschopp
- National Research Centre Frontiers in Genetics at Department of Genetics and Evolution, University of Geneva, Sciences III, Quai Ernest-Ansermet 30, Geneva 4, Switzerland
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16
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Sabharwal P, Lee C, Park S, Rao M, Sockanathan S. GDE2 regulates subtype-specific motor neuron generation through inhibition of Notch signaling. Neuron 2011; 71:1058-70. [PMID: 21943603 DOI: 10.1016/j.neuron.2011.07.028] [Citation(s) in RCA: 37] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 07/29/2011] [Indexed: 12/29/2022]
Abstract
The specification of spinal interneuron and motor neuron identities initiates within progenitor cells, while motor neuron subtype diversification is regulated by hierarchical transcriptional programs implemented postmitotically. Here we find that mice lacking GDE2, a six-transmembrane protein that triggers motor neuron generation, exhibit selective losses of distinct motor neuron subtypes, specifically in defined subsets of limb-innervating motor pools that correlate with the loss of force-generating alpha motor neurons. Mechanistically, GDE2 is expressed by postmitotic motor neurons but utilizes extracellular glycerophosphodiester phosphodiesterase activity to induce motor neuron generation by inhibiting Notch signaling in neighboring motor neuron progenitors. Thus, neuronal GDE2 controls motor neuron subtype diversity through a non-cell-autonomous feedback mechanism that directly regulates progenitor cell differentiation, implying that subtype specification initiates within motor neuron progenitor populations prior to their differentiation into postmitotic motor neurons.
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Affiliation(s)
- Priyanka Sabharwal
- The Solomon Snyder Department of Neuroscience, The Johns Hopkins University School of Medicine, PCTB1004, 725 N. Wolfe Street, Baltimore, MD 21205, USA
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17
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Murakami Y, Tanaka M. Evolution of motor innervation to vertebrate fins and limbs. Dev Biol 2011; 355:164-72. [DOI: 10.1016/j.ydbio.2011.04.009] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/04/2010] [Revised: 04/08/2011] [Accepted: 04/13/2011] [Indexed: 01/04/2023]
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18
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MicroRNA miR-9 modifies motor neuron columns by a tuning regulation of FoxP1 levels in developing spinal cords. J Neurosci 2011; 31:809-18. [PMID: 21248104 DOI: 10.1523/jneurosci.4330-10.2011] [Citation(s) in RCA: 100] [Impact Index Per Article: 7.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
The precise organization of motor neuron subtypes in a columnar pattern in developing spinal cords is controlled by cross-interactions of multiple transcription factors and segmental expressions of Hox genes and their accessory proteins. Accurate expression levels and domains of these regulators are essential for organizing spinal motor neuron columns and axonal projections to target muscles. Here, we show that microRNA miR-9 is transiently expressed in a motor neuron subtype and displays overlapping expression with its target gene FoxP1. Overexpression or knockdown of miR-9 alters motor neuron subtypes, switches columnar identities, and changes axonal innervations in developing chick spinal cords. miR-9 modifies spinal columnar organization by specifically regulating FoxP1 protein levels, which in turn determine distinct motor neuron subtypes. Our findings demonstrate that miR-9 is an essential regulator of motor neuron specification and columnar formation. Moreover, the overlapping expression of miR-9 and its target FoxP1 further illuminates the importance of fine-tuning regulation by microRNAs in motor neuron development.
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19
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Jung H, Lacombe J, Mazzoni EO, Liem KF, Grinstein J, Mahony S, Mukhopadhyay D, Gifford DK, Young RA, Anderson KV, Wichterle H, Dasen JS. Global control of motor neuron topography mediated by the repressive actions of a single hox gene. Neuron 2010; 67:781-96. [PMID: 20826310 DOI: 10.1016/j.neuron.2010.08.008] [Citation(s) in RCA: 91] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 07/23/2010] [Indexed: 10/19/2022]
Abstract
In the developing spinal cord, regional and combinatorial activities of Hox transcription factors are critical in controlling motor neuron fates along the rostrocaudal axis, exemplified by the precise pattern of limb innervation by more than fifty Hox-dependent motor pools. The mechanisms by which motor neuron diversity is constrained to limb levels are, however, not well understood. We show that a single Hox gene, Hoxc9, has an essential role in organizing the motor system through global repressive activities. Hoxc9 is required for the generation of thoracic motor columns, and in its absence, neurons acquire the fates of limb-innervating populations. Unexpectedly, multiple Hox genes are derepressed in Hoxc9 mutants, leading to motor pool disorganization and alterations in the connections by thoracic and forelimb-level subtypes. Genome-wide analysis of Hoxc9 binding suggests that this mode of repression is mediated by direct interactions with Hox regulatory elements, independent of chromatin marks typically associated with repressed Hox genes.
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Affiliation(s)
- Heekyung Jung
- Howard Hughes Medical Institute, Smilow Neuroscience Program, Department of Physiology and Neuroscience, New York University School of Medicine, New York, NY 10016, USA
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20
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Takahashi Y, Ohtori S, Takahashi K. Somatotopic organization of lumbar muscle-innervating neurons in the ventral horn of the rat spinal cord. J Anat 2010; 216:489-95. [PMID: 20136668 DOI: 10.1111/j.1469-7580.2009.01203.x] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022] Open
Abstract
The ventral horn of the rat spinal cord was investigated with respect to the somatotopic organization of the motor neurons that innervate the lumbar muscles. Neurotracer 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) was applied to specific sites in lumbar muscles. Spinal cord segments at L1 through L4 levels were cut into 40-mum serial transverse sections. Labeled neurons were located in the ventromedial nucleus (VM) and lateromedial nucleus (LM) nuclei of Rexed's lamina IX. Motor neurons innervating the m. interspinales lumborum and m. multifidus were without exception present in the VM, whereas all motor neurons innervating the m. rectus abdominis were present in the LM. Forty percent of motor neurons innervating the m. quadratus lumborum were present in the VM and the other 60% were in the LM. Although most of the motor neurons innervating the m. psoas major were present in the LM, a few labeled neurons existed in the VM. These results suggest that the border zone demarcating the areas of innervation of the dorsal and ventral rami of spinal nerves crosses the m. quadratus lumborum.
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Affiliation(s)
- Yuzuru Takahashi
- Department of Orthopaedic Surgery, Graduate School of Medicine, Chiba University, Chiba, Japan.
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21
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Dasen JS, Jessell TM. Chapter Six Hox Networks and the Origins of Motor Neuron Diversity. Curr Top Dev Biol 2009; 88:169-200. [DOI: 10.1016/s0070-2153(09)88006-x] [Citation(s) in RCA: 248] [Impact Index Per Article: 16.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023]
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22
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Dasen JS, De Camilli A, Wang B, Tucker PW, Jessell TM. Hox Repertoires for Motor Neuron Diversity and Connectivity Gated by a Single Accessory Factor, FoxP1. Cell 2008; 134:304-16. [PMID: 18662545 DOI: 10.1016/j.cell.2008.06.019] [Citation(s) in RCA: 275] [Impact Index Per Article: 17.2] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/12/2008] [Revised: 05/06/2008] [Accepted: 06/12/2008] [Indexed: 12/28/2022]
Affiliation(s)
- Jeremy S Dasen
- Smilow Neuroscience Program, Department of Physiology and Neuroscience, New York University School of Medicine, New York, NY 10016, USA.
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23
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Moret F, Renaudot C, Bozon M, Castellani V. Semaphorin and neuropilin co-expression in motoneurons sets axon sensitivity to environmental semaphorin sources during motor axon pathfinding. Development 2008; 134:4491-501. [PMID: 18039974 DOI: 10.1242/dev.011452] [Citation(s) in RCA: 74] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Class III semaphorins (SemaIIIs) are intercellular cues secreted by surrounding tissues to guide migrating cells and axons in the developing organism. This chemotropic activity is crucial for the formation of nerves and vasculature. Intriguingly, SemaIIIs are also synthesized by neurons during axon pathfinding, but their function as intrinsic cues remains unknown. We have explored the role of Sema3A expression in motoneurons during spinal nerve development. Loss- and gain-of-function in the neural tube of the chick embryo were undertaken to target Sema3A expression in motoneurons while preserving Sema3A sources localized in peripheral tissues, known to provide important repulsive information for delineating the routes of motor axons towards their ventral or dorsal targets. Strikingly, Sema3A overexpression induced defasciculation and exuberant growth of motor axon projections into these normally non-permissive territories. Moreover, knockdown studies showed that motoneuronal Sema3A is required for correct spinal nerve compaction and dorsal motor axon extension. Further analysis of Sema3A gain- and loss-of-function in ex vivo models revealed that Sema3A in motoneurons sets the level of sensitivity of their growth cones to exogenous Sema3A exposure. This regulation is associated with post-transcriptional and local control of the availability of the Sema3A receptor neuropilin 1 at the growth cone surface. Thus, by modulating the strength of Sema3A-mediated environmental repulsive constraints, Sema3A in motoneurons enables axons to extend more or less far away from these repulsive sources. Such interplay between intrinsic and extrinsic Sema3A may represent a fundamental mechanism in the accurate specification of axon pathways.
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24
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Ji SJ, Zhuang B, Falco C, Schneider A, Schuster-Gossler K, Gossler A, Sockanathan S. Mesodermal and neuronal retinoids regulate the induction and maintenance of limb innervating spinal motor neurons. Dev Biol 2006; 297:249-61. [PMID: 16781703 DOI: 10.1016/j.ydbio.2006.05.015] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/04/2006] [Revised: 04/13/2006] [Accepted: 05/11/2006] [Indexed: 11/25/2022]
Abstract
During embryonic development, the generation, diversification and maintenance of spinal motor neurons depend upon extrinsic signals that are tightly regulated. Retinoic acid (RA) is necessary for specifying the fates of forelimb-innervating motor neurons of the Lateral Motor Column (LMC), and the specification of LMC neurons into medial and lateral subtypes. Previous studies implicate motor neurons as the relevant source of RA for specifying lateral LMC fates at forelimb levels. However, at the time of LMC diversification, a significant amount of retinoids in the spinal cord originates from the adjacent paraxial mesoderm. Here we employ mouse genetics to show that RA derived from the paraxial mesoderm is required for lateral LMC induction at forelimb and hindlimb levels, demonstrating that mesodermally synthesized RA functions as a second source of signals to specify lateral LMC identity. Furthermore, reduced RA levels in postmitotic motor neurons result in a decrease of medial and lateral LMC neurons, and abnormal axonal projections in the limb; invoking additional roles for neuronally synthesized RA in motor neuron maintenance and survival. These findings suggest that during embryogenesis, mesodermal and neuronal retinoids act coordinately to establish and maintain appropriate cohorts of spinal motor neurons that innervate target muscles in the limb.
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Affiliation(s)
- Sheng-Jian Ji
- Department of Neuroscience, Johns Hopkins University School of Medicine, 725 N Wolfe Street, Baltimore, MD 21205, USA
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25
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Wilson L, Maden M. The mechanisms of dorsoventral patterning in the vertebrate neural tube. Dev Biol 2005; 282:1-13. [PMID: 15936325 DOI: 10.1016/j.ydbio.2005.02.027] [Citation(s) in RCA: 211] [Impact Index Per Article: 11.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/17/2004] [Revised: 02/21/2005] [Accepted: 02/25/2005] [Indexed: 01/03/2023]
Abstract
We describe the essential features of and the molecules involved in dorsoventral (DV) patterning in the neural tube. The neural tube is, from its very outset, patterned in this axis as there is a roof plate, floor plate, and differing numbers and types of neuroblasts. These neuroblasts develop into different types of neurons which express a different range of marker genes. Early embryological experiments identified the notochord and the somites as being responsible for the DV patterning of the neural tube and we now know that 4 signaling molecules are involved and are generated by these surrounding structures. Fibroblast growth factors (FGFs) are produced by the caudal mesoderm and must be down-regulated before neural differentiation can occur. Retinoic acid (RA) is produced by the paraxial mesoderm and is an inducer of neural differentiation and patterning and is responsible for down-regulating FGF. Sonic hedgehog (Shh) is produced by the notochord and floor plate and is responsible for inducing ventral neural cell types in a concentration-dependent manner. Bone morphogenetic proteins (BMPs) are produced by the roof plate and are responsible for inducing dorsal neural cell types in a concentration-dependent manner. Subsequently, RA is used twice more. Once from the somites for motor neuron differentiation and secondly RA is used to define the motor neuron subtypes, but in the latter case it is generated within the neural tube from differentiating motor neurons rather than from outside. These 4 signaling molecules also interact with each other, generally in a repressive fashion, and DV patterning shows how complex these interactions can be.
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Affiliation(s)
- Leigh Wilson
- MRC Centre for Developmental Neurobiology, 4th Floor New Hunt's House, King's College London, Guy's Campus, London SE1 1UL, UK
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26
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Price SR, Briscoe J. The generation and diversification of spinal motor neurons: signals and responses. Mech Dev 2005; 121:1103-15. [PMID: 15296975 DOI: 10.1016/j.mod.2004.04.019] [Citation(s) in RCA: 45] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/06/2004] [Revised: 04/26/2004] [Accepted: 04/26/2004] [Indexed: 11/30/2022]
Abstract
Motor neurons are probably the best characterised neuronal class in the vertebrate central nervous system and have become a paradigm for understanding the mechanisms that control the development of vertebrate neurons. For many investigators working on this problem the chick embryo is the model system of choice and from these studies a picture of the steps involved in motor neuron generation has begun to emerge. These findings suggest that motor neuron generation is shaped by extracellular signals that regulate intrinsic, cell-autonomous determinants at sequential steps during development. The chick embryo has played a prominent role in identifying the sources of these signals, defining their molecular identities and determining the cell intrinsic programs they regulate.
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Affiliation(s)
- Stephen R Price
- Department of Anatomy and Developmental Biology, University College London, Gower Street, London WC1E 6BT, UK.
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27
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Shah V, Drill E, Lance-Jones C. Ectopic expression of Hoxd10 in thoracic spinal segments induces motoneurons with a lumbosacral molecular profile and axon projections to the limb. Dev Dyn 2005; 231:43-56. [PMID: 15305286 DOI: 10.1002/dvdy.20103] [Citation(s) in RCA: 48] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022] Open
Abstract
Hox genes encode anterior-posterior identity during central nervous system development. Few studies have examined Hox gene function at lumbosacral (LS) levels of the spinal cord, where there is extensive information on normal development. Hoxd10 is expressed at high levels in the embryonic LS cord but not the thoracic cord. To test the hypothesis that restricted expression of Hoxd10 contributes to the attainment of an LS identity, and specifically an LS motoneuron identity, Hoxd10 was ectopically expressed in thoracic segments in chick embryos by means of in ovo electroporation. Regional motoneuron identity was assessed after the normal period of motoneuron differentiation. Subsets of motoneurons in transfected thoracic segments developed a molecular profile normally shown by LS motoneurons, including Lim 1 and RALDH2 expression. In addition, motoneurons in posterior thoracic segments showed novel axon projections to two muscles in the anterodorsal limb, the sartorius and anterior iliotibialis muscles. At thoracic levels, we also found a decrease in motoneuron numbers and a reduction in gonad size. These last findings suggest that early and high levels of Hox expression impeded motoneuron development and neural-mesodermal interactions. Despite these adverse effects, our data indicate that Hoxd10 expression is sufficient to induce LS motoneuron identity and axon trajectories characteristic of motoneurons in the LS region.
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Affiliation(s)
- Veeral Shah
- Department of Neurobiology, Center for Neuroscience, University of Pittsburgh, Pittsburgh, Pennsylvania, USA
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28
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Dasen JS, Liu JP, Jessell TM. Motor neuron columnar fate imposed by sequential phases of Hox-c activity. Nature 2003; 425:926-33. [PMID: 14586461 DOI: 10.1038/nature02051] [Citation(s) in RCA: 262] [Impact Index Per Article: 12.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/22/2003] [Accepted: 09/03/2003] [Indexed: 01/07/2023]
Abstract
The organization of neurons into columns is a prominent feature of central nervous system structure and function. In many regions of the central nervous system the grouping of neurons into columns links cell-body position to axonal trajectory, thus contributing to the establishment of topographic neural maps. This link is prominent in the developing spinal cord, where columnar sets of motor neurons innervate distinct targets in the periphery. We show here that sequential phases of Hox-c protein expression and activity control the columnar differentiation of spinal motor neurons. Hox expression in neural progenitors is established by graded fibroblast growth factor signalling and translated into a distinct motor neuron Hox pattern. Motor neuron columnar fate then emerges through cell autonomous repressor and activator functions of Hox proteins. Hox proteins also direct the expression of genes that establish motor topographic projections, thus implicating Hox proteins as critical determinants of spinal motor neuron identity and organization.
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Affiliation(s)
- Jeremy S Dasen
- Howard Hughes Medical Institute, Department of Biochemistry and Molecular Biophysics, Center for Neurobiology and Behavior, Columbia University, 701 West 168th Street, New York, New York 10032, USA.
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29
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Sockanathan S, Perlmann T, Jessell TM. Retinoid Receptor Signaling in Postmitotic Motor Neurons Regulates Rostrocaudal Positional Identity and Axonal Projection Pattern. Neuron 2003; 40:97-111. [PMID: 14527436 DOI: 10.1016/s0896-6273(03)00532-4] [Citation(s) in RCA: 106] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023]
Abstract
The identity of motor neurons diverges markedly at different rostrocaudal levels of the spinal cord, but the signals that specify their fate remain poorly defined. We show that retinoid receptor activation in newly generated spinal motor neurons has a crucial role in specifying motor neuron columnar subtypes. Blockade of retinoid receptor signaling in brachial motor neurons inhibits lateral motor column differentiation and converts many of these neurons to thoracic columnar subtypes. Conversely, expression of a constitutively active retinoid receptor derivative impairs the differentiation of thoracic motor neuron columnar subtypes. These findings provide evidence for a regionally restricted role for retinoid signaling in the postmitotic specification of motor neuron columnar identity.
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Affiliation(s)
- Shanthini Sockanathan
- Howard Hughes Medical Institute, Department of Biochemistry and Molecular Biophysics, Center for Neurobiology and Behavior, Columbia University, New York, NY 10032, USA
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30
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Coonan JR, Bartlett PF, Galea MP. Role of EphA4 in defining the position of a motoneuron pool within the spinal cord. J Comp Neurol 2003; 458:98-111. [PMID: 12577325 DOI: 10.1002/cne.10571] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
The correct assembly of the neural circuits that control movement requires the development of topographically organized pools of motoneurons within the spinal cord. The generation of a diverse array of motoneuron subtypes, which express differing transcription factors and cell-surface receptors, allows different motoneuron pools to be segregated to specific positions during development. In this investigation, we show that the Eph receptor tyrosine kinase, EphA4, appears to be important for the correct localization of a motoneuron pool to a specific position in the spinal cord. In the spinal cord of mice deficient in EphA4, the motoneuron pool that innervates the tibialis anterior muscle of the hindlimb is caudally displaced by approximately one vertebral segment. However, despite the abnormal position of the tibialis anterior motoneuron pool in the spinal cord of EphA4-deficient animals, the motoneurons of this pool still project to the tibialis anterior muscle of the hindlimb correctly. Additional analyses of other limb innervating motoneuron pools in the cervical and lumbar enlargements of the spinal cord of EphA4-deficient animals revealed them to be located in the appropriate segmental positions. Furthermore, we show that EphA4 does not appear to be important for spinal motoneuron survival as stereological quantification of the number of motoneurons present in the sciatic motoneuron pool of EphA4-deficient animals demonstrated these motoneurons to be present in the correct numbers. These observations suggest an important role for EphA4 in regulating the position of a specific motoneuron pool within the spinal cord.
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Affiliation(s)
- Jason R Coonan
- The Walter and Eliza Hall Institute of Medical Research, The Royal Melbourne Hospital, Victoria 3050, Australia
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31
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Schubert W, Kaprielian Z. Identification and characterization of a cell surface marker for embryonic rat spinal accessory motor neurons. J Comp Neurol 2001; 439:368-83. [PMID: 11596060 DOI: 10.1002/cne.1356] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
The developing mammalian spinal cord contains distinct populations of motor neurons that can be distinguished by their cell body positions, by the expression of specific combinations of regulatory genes, and by the paths that their axons take to exit the central nervous system (CNS). Subclasses of spinal motor neurons are also thought to express specific cell surface proteins that function as receptors which control the guidance of their axons. We identified monoclonal antibody (mAb) SAC1 in a screen aimed at generating markers for specific subsets of neurons/axons in the developing rat spinal cord. During early embryogenesis, mAb SAC1 selectively labels a small subset of Isl1-positive motor neurons located exclusively within cervical segments of the spinal cord. Strikingly, these neurons extend mAb SAC1-positive axons along a dorsally directed trajectory toward the lateral exit points. Consistent with the finding that mAb SAC1 also labels spinal accessory nerves, these observations identify mAb SAC1 as a specific marker of spinal accessory motor neurons/axons. During later stages of embryogenesis, mAb SAC1 is transiently expressed on both dorsally and ventrally projecting spinal motor neurons/axons. Interestingly, mAb SAC1 also labels the notochord and floor plate during most stages of spinal cord development. The mAb SAC1 antigen is a 100-kD glycoprotein that is likely to be the rat homolog of SC1/BEN/DM-GRASP, a homophilic adhesion molecule that mediates axon outgrowth and fasciculation.
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Affiliation(s)
- W Schubert
- Department of Neuroscience, Albert Einstein College of Medicine, Bronx, New York 10461, USA
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32
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Yaginuma H, Shiraiwa N, Shimada T, Nishiyama K, Hong J, Wang S, Momoi T, Uchiyama Y, Oppenheim RW. Caspase activity is involved in, but is dispensable for, early motoneuron death in the chick embryo cervical spinal cord. Mol Cell Neurosci 2001; 18:168-82. [PMID: 11520178 DOI: 10.1006/mcne.2001.1009] [Citation(s) in RCA: 40] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
We examined the role of caspases in the early programmed cell death (PCD) of motoneurons (MNs) in the chick embryo cervical cord between embryonic day (E) 4 and E5. An increase in caspase-3-like activity in MNs was observed at E4.5. Treatment with an inhibitor of caspase-3-like activity, Ac-DEVD-CHO, for 12 h blocked this increase and revealed that caspase-3-like activity is mainly responsible for DNA fragmentation and the nuclear changes during PCD but not for degenerative changes in the cytoplasm. When a more broad-spectrum caspase inhibitor was used (bocaspartyl (OMe)-fluoromethyl ketone, BAF), the appearance of degenerative changes in the cytoplasm was delayed by at least 12 h. However, following treatment with either Ac-DEVD-CHO or BAF for 24 h, the number of surviving healthy MNs did not differ from controls, indicating a normal occurrence of PCD despite the inhibition of caspases. These results suggest that caspase cascades that occur upstream of and are independent of the activation of caspase-3-like activity are responsible for the degenerative changes in the cytoplasm of dying cervical MNs. These data also suggest that, although one function of caspases may be to facilitate the kinetics of PCD, caspases are nonetheless dispensable for at least some forms of normal neuronal PCD in vivo.
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Affiliation(s)
- H Yaginuma
- Department of Anatomy, Fukushima Medical University, Fukushima 960-1295, Japan.
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33
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Gould TW, Burek MJ, Sosnowski JM, Prevette D, Oppenheim RW. The spatial-temporal gradient of naturally occurring motoneuron death reflects the time of prior exit from the cell cycle and position within the lateral motor column. Dev Biol 1999; 216:611-21. [PMID: 10642796 DOI: 10.1006/dbio.1999.9490] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/03/2023]
Abstract
Embryonic lumbar spinal motoneurons (MNs) are characterized by a period of programmed cell death (PCD) that spans several days and occurs in a rostrocaudal gradient. The generation of these MNs also takes place in a temporal-spatial gradient, such that MNs within rostral lumbar segments exit the cell cycle earlier and MNs within progressively caudal regions are born later. In vitro studies have shown that the latest born spinal MNs, presumably through the possession of endogenous "survival properties," are also the last to acquire their trophic dependence. If the birth date and therefore spinal cord location of lumbar spinal MNs influence the spatial-temporal pattern of PCD, then earlier born MNs should die sooner and be located more rostrally than those generated later. Alternatively, if the time at which MNs die during development is unrelated to their prior exit from the cell cycle, those born at various phases should die throughout the period of PCD. We report here that lumbar MNs generated during the earliest part (embryonic day 2-3) of the proliferative period in the developing chick spinal cord tend to die during the earliest stages of the PCD period and that MNs born in successive 12-h intervals die at correspondingly later periods during PCD. Furthermore, the spatial progression of PCD of these subpopulations of MNs occurs in a rostrocaudal gradient. Finally, while MNs do appear to die in a mediolateral gradient during the period of MN PCD, this pattern is only partly accounted for by MNs born in consecutive intervals. These data support the notion that the timing and rostrocaudal location of MNs undergoing PCD reflect their time of exit from the cell cycle.
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Affiliation(s)
- T W Gould
- Department of Neurobiology and Anatomy, Wake Forest University School of Medicine, Winston-Salem, North Carolina 27157, USA
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34
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Saueressig H, Burrill J, Goulding M. Engrailed-1 and netrin-1 regulate axon pathfinding by association interneurons that project to motor neurons. Development 1999; 126:4201-12. [PMID: 10477289 DOI: 10.1242/dev.126.19.4201] [Citation(s) in RCA: 112] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
During early development, multiple classes of interneurons are generated in the spinal cord including association interneurons that synapse with motor neurons and regulate their activity. Very little is known about the molecular mechanisms that generate these interneuron cell types, nor is it known how axons from association interneurons are guided toward somatic motor neurons. By targeting the axonal reporter gene τ-lacZ to the En1 locus, we show the cell-type-specific transcription factor Engrailed-1 (EN1) defines a population of association neurons that project locally to somatic motor neurons. These EN1 interneurons are born early and their axons pioneer an ipsilateral longitudinal projection in the ventral spinal cord. The EN1 interneurons extend axons in a stereotypic manner, first ventrally, then rostrally for one to two segments where their axons terminate close to motor neurons. We show that the growth of EN1 axons along a ventrolateral pathway toward motor neurons is dependent on netrin-1 signaling. In addition, we demonstrate that En1 regulates pathfinding and fasciculation during the second phase of EN1 axon growth in the ventrolateral funiculus (VLF); however, En1 is not required for the early specification of ventral interneuron cell types in the embryonic spinal cord.
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Affiliation(s)
- H Saueressig
- Molecular Neurobiology Laboratory, The Salk Institute, La Jolla, CA 92037, USA
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35
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Ensini M, Tsuchida TN, Belting HG, Jessell TM. The control of rostrocaudal pattern in the developing spinal cord: specification of motor neuron subtype identity is initiated by signals from paraxial mesoderm. Development 1998; 125:969-82. [PMID: 9463344 DOI: 10.1242/dev.125.6.969] [Citation(s) in RCA: 133] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
Abstract
The generation of distinct classes of motor neurons is an early step in the control of vertebrate motor behavior. To study the interactions that control the generation of motor neuron subclasses in the developing avian spinal cord we performed in vivo grafting studies in which either the neural tube or flanking mesoderm were displaced between thoracic and brachial levels. The positional identity of neural tube cells and motor neuron subtype identity was assessed by Hox and LIM homeodomain protein expression. Our results show that the rostrocaudal identity of neural cells is plastic at the time of neural tube closure and is sensitive to positionally restricted signals from the paraxial mesoderm. Such paraxial mesodermal signals appear to control the rostrocaudal identity of neural tube cells and the columnar subtype identity of motor neurons. These results suggest that the generation of motor neuron subtypes in the developing spinal cord involves the integration of distinct rostrocaudal and dorsoventral patterning signals that derive, respectively, from paraxial and axial mesodermal cell groups.
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Affiliation(s)
- M Ensini
- Howard Hughes Medical Institute, Department of Biochemistry and Molecular Biophysics, Columbia University, New York, NY 10025, USA
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36
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Abstract
Thymocyte winged helix (TWH) is a putative transcription factor expressed in the developing neural tube. At midgestation, TWH expression identifies subsets of spinal cord motor neurons and interneurons. TWH-expressing motor neurons were restricted to specific spinal cord levels, distinguishing motor neurons at lumbar from those at cervical levels. To understand the developmental role of TWH, we replaced the TWH gene with the lacZ reporter gene and generated mice with a homozygous disruption of the TWH gene. TWH(-/-) mutant mice had increased perinatal mortality, retarded postnatal growth, and motor weakness. The TWH(-/-) mutation resulted in alterations in the sizes and position of different neuronal populations. Our results demonstrate that TWH plays a critical role in neuronal development and suggest that TWH regulates the early differentiation of neural progenitors.
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Affiliation(s)
- C Dou
- Cell Biology Program and Division of Endocrinology, Memorial Sloan-Kettering Cancer Center, New York, New York 10021, USA
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37
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Appel B, Korzh V, Glasgow E, Thor S, Edlund T, Dawid IB, Eisen JS. Motoneuron fate specification revealed by patterned LIM homeobox gene expression in embryonic zebrafish. Development 1995; 121:4117-25. [PMID: 8575312 DOI: 10.1242/dev.121.12.4117] [Citation(s) in RCA: 201] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
In zebrafish, individual primary motoneurons can be uniquely identified by their characteristic cell body positions and axonal projection patterns. The fate of individual primary motoneurons remains plastic until just prior to axogenesis when they become committed to particular identities. We find that distinct primary motoneurons express particular combinations of LIM homeobox genes. Expression precedes axogenesis as well as commitment, suggesting that LIM homeobox genes may contribute to the specification of motoneuronal fates. By transplanting them to new spinal cord positions, we demonstrate that primary motoneurons can initiate a new program of LIM homeobox gene expression, as well as the morphological features appropriate for the new position. We conclude that the patterned distribution of different primary motoneuronal types within the zebrafish spinal cord follows the patterned expression of LIM homeobox genes, and that this reflects a highly resolved system of positional information controlling gene transcription.
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Affiliation(s)
- B Appel
- Institute of Neuroscience, University of Oregon, Eugene 97403, USA
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38
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Tsuchida T, Ensini M, Morton SB, Baldassare M, Edlund T, Jessell TM, Pfaff SL. Topographic organization of embryonic motor neurons defined by expression of LIM homeobox genes. Cell 1994; 79:957-70. [PMID: 7528105 DOI: 10.1016/0092-8674(94)90027-2] [Citation(s) in RCA: 874] [Impact Index Per Article: 29.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
Abstract
Motor neurons located at different positions in the embryonic spinal cord innervate distinct targets in the periphery, establishing a topographic neural map. The topographic organization of motor projections depends on the generation of subclasses of motor neurons that select specific paths to their targets. We have cloned a family of LIM homeobox genes in chick and show here that the combinatorial expression of four of these genes, Islet-1, Islet-2, Lim-1, and Lim-3, defines subclasses of motor neurons that segregate into columns in the spinal cord and select distinct axonal pathways. These genes are expressed prior to the formation of distinct motor axon pathways and before motor columns appear. Our results suggest that LIM homeobox genes contribute to the generation of motor neuron diversity and may confer subclasses of motor neurons with the ability to select specific axon pathways, thereby initiating the topographic organization of motor projections.
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Affiliation(s)
- T Tsuchida
- Howard Hughes Medical Institute Department of Biochemistry and Molecular Biophysics, Columbia University, New York, New York 10032
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