ABSTRACT
Unravelling how neurons are guided during vertebrate embryonic development has wide implications for understanding the assembly of the nervous system. During embryogenesis, migration of neuronal cell bodies and axons occurs simultaneously, but to what degree they influence each other's development remains obscure. We show here that within the mouse embryonic spinal cord, commissural axons bisect, delimit or preconfigure ventral interneuron cell body position. Furthermore, genetic disruption of commissural axons results in abnormal ventral interneuron cell body positioning. These data suggest that commissural axonal fascicles instruct cell body position by acting either as border landmarks (axon-restricted migration), which to our knowledge has not been previously addressed, or acting as cellular guides. This study in the developing spinal cord highlights an important function for the interaction of cell bodies and axons, and provides a conceptual proof of principle that is likely to have overarching implications for the development of neuronal architecture.
Subject(s)
Cell Movement/physiology , Commissural Interneurons/physiology , Neurons/physiology , Spinal Cord/cytology , Spinal Cord/embryology , Animals , Cell Body , Cells, Cultured , Chick Embryo , DCC Receptor , Electroporation , Female , Gene Expression Regulation/physiology , Male , Membrane Proteins/genetics , Membrane Proteins/metabolism , Mice , Nerve Growth Factors/genetics , Nerve Growth Factors/metabolism , Nerve Tissue Proteins/genetics , Nerve Tissue Proteins/metabolism , Netrin-1 , Plasmids , Receptors, Cell Surface/genetics , Receptors, Cell Surface/metabolism , Reverse Transcriptase Polymerase Chain Reaction , Tumor Suppressor Proteins/genetics , Tumor Suppressor Proteins/metabolismABSTRACT
The establishment of anatomically stereotyped axonal projections is fundamental to neuronal function. While most neurons project their axons within the central nervous system (CNS), only axons of centrally born motoneurons and peripherally born sensory neurons link the CNS and peripheral nervous system (PNS) together by navigating through specialized CNS/PNS transition zones. Such selective restriction is of importance because inappropriate CNS axonal exit could lead to loss of correct connectivity and also to gain of erroneous functions. However, to date, surprisingly little is known about the molecular-genetic mechanisms that regulate how central axons are confined within the CNS during development. Here, we show that netrin 1/Dcc/Unc5 chemotropism contributes to axonal confinement within the CNS. In both Ntn1 and Dcc mutant mouse embryos, some spinal interneuronal axons exit the CNS by traversing the CNS/PNS transition zones normally reserved for motor and sensory axons. We provide evidence that netrin 1 signalling preserves CNS/PNS axonal integrity in three ways: (1) netrin 1/Dcc ventral attraction diverts axons away from potential exit points; (2) a Dcc/Unc5c-dependent netrin 1 chemoinhibitory barrier in the dorsolateral spinal cord prevents interneurons from being close to the dorsal CNS/PNS transition zone; and (3) a netrin 1/Dcc-dependent, Unc5c-independent mechanism that actively prevents exit from the CNS. Together, these findings provide insights into the molecular mechanisms that maintain CNS/PNS integrity and, to the best of our knowledge, present the first evidence that chemotropic signalling regulates interneuronal CNS axonal confinement in vertebrates.
Subject(s)
Axons/metabolism , Central Nervous System/metabolism , Nerve Growth Factors/metabolism , Receptors, Cell Surface/metabolism , Signal Transduction , Tumor Suppressor Proteins/metabolism , Animals , Biomarkers/metabolism , Central Nervous System/cytology , DCC Receptor , Embryo, Mammalian/cytology , Embryo, Mammalian/metabolism , Gene Expression Regulation, Developmental , Interneurons/cytology , Interneurons/metabolism , Membrane Proteins/metabolism , Mice , Mice, Inbred C57BL , Models, Biological , Nerve Growth Factors/deficiency , Nerve Growth Factors/genetics , Nerve Tissue Proteins/metabolism , Netrin-1 , Peripheral Nervous System/cytology , Peripheral Nervous System/metabolism , RNA, Messenger/genetics , RNA, Messenger/metabolism , Receptors, Cell Surface/deficiency , Signal Transduction/genetics , Spinal Cord/cytology , Spinal Cord/embryology , Tumor Suppressor Proteins/deficiency , Tumor Suppressor Proteins/genetics , beta-Galactosidase/metabolismABSTRACT
BACKGROUND: Neuronal circuit assembly comprises a number of developmental processes that ultimately underlie function. Identifying the molecular events that dictate these processes can give key insights into how neuronal circuit formation is coordinated. To begin to identify such molecular mechanisms, we have analysed the expression of a candidate gene of entirely unknown function within the nervous system. Here we reveal the spatial and temporal distribution of Lzts1 in mouse and chick embryonic spinal cord and propose potential biological functions. RESULTS: Lzts1 mRNA is transiently expressed at the border of the ventricular and mantle zones in subsets of sensory and motor spinal neurons. The protein is localized to the cell body, axon, and trailing process of motor, commissural, and dorsal root neurons during development. CONCLUSIONS: Taken together, the spatial and temporal distribution of Lzts1 is consistent with a potential function(s) in cell cycle regulation, axon growth or guidance, and/or migration of neurons.
Subject(s)
DNA-Binding Proteins/genetics , Gene Expression Regulation, Developmental , Neurons/metabolism , Spinal Cord/embryology , Tumor Suppressor Proteins/genetics , Animals , Axons/metabolism , Chick Embryo , DNA-Binding Proteins/metabolism , Leucine Zippers , Mice , Neurogenesis , Neurons/cytology , Spinal Cord/cytology , Spinal Cord/metabolism , Tumor Suppressor Proteins/metabolismABSTRACT
Despite increasing evidence for transcriptional control of neural connectivity, how transcription factors regulate discrete steps in axon guidance remains obscure. Projection neurons in the dorsal spinal cord relay sensory signals to higher brain centers. Some projection neurons send their axons ipsilaterally, whereas others, commissural neurons, send axons contralaterally. We show that two closely related LIM homeodomain proteins, Lhx2 and Lhx9, are expressed by a set of commissural relay neurons (dI1c neurons) and are required for the dI1c axon projection. Midline crossing by dI1c axons is lost in Lhx2/9 double mutants, a defect that results from loss of expression of Rig-1 from dI1c axons. Lhx2 binds to a conserved motif in the Rig-1 gene, suggesting that Lhx2/9 regulate directly the expression of Rig-1. Our findings reveal a link between the transcriptional programs that define neuronal subtype identity and the expression of receptors that guide distinctive aspects of their trajectory.
