This SnapShot outlines the sequential genetic steps that generate neuronal diversity within an idealized spinal segment of the mouse. The progression from neural progenitor cells to postmitotic neurons spanning embryonic day 9.5 (e9.5) to e18.5 is shown from left to right, although some events are not strictly linear. Diverse combinations of Hox transcription factor expression along the rostrocaudal (i.e., head-to-tail) axis further subdivide motor neurons, but for clarity, these patterns are not reflected in this idealized segment. Recent studies have begun to define the functions of the cardinal cell types in the spinal cord, particularly those that relate to locomotor behaviors.
Precursor Generation
Cellular identities are defined by the influence of a two-dimensional coordinate system of morphogen gradients that act on the neuroepithelial cells occupying the ventricular zone of the ~e9.5 neural tube. A Sonic hedgehog (Shh) gradient produced by the notochord and the floor plate establishes the identity of five ventral progenitor cell domains (p0, p1, p2, pMN, and p3), marked by the expression transcription factors with a basic-helix-loop-helix (bHLH) domain and a homeodomain. Genes repressed by Shh are categorized as class I (e.g., Irx3), and genes induced by Shh are termed class II (e.g., Olig2). Typically, the transcription factors in adjacent progenitor domains repress expression of factors in neighboring domains, preventing cells from developing with hybrid identities. Transforming growth factor β (TGFβ) family proteins from the overlying ectoderm (e.g., Bmp4) and roof plate (e.g., Gdnf7) produces dorsalizing signals. The dorsal-most progenitor domains, pd1–pd3, depend on TGFβ, whereas pd4–pd6 and pdIL are independent of TGFβ. The somite produces retinoic acid (RA) that controls subtype and dorsoventral identity through Pax6. Within this idealized segment of spinal cord, these morphogen gradients establish 12 progenitor domains that grossly give rise to seven dorsal interneuron progenitor divisions, pd1–6 and pdIL; four ventral interneuron progenitor divisions, p0–3; and one motor neuron progenitor domain, pMN. Ventral progenitor domains (e.g., pMN) produce one cell type early (e.g., motor neurons) followed by another cell type later (e.g., oligodendrocytes).
Refinement and Subdivision of Classes
As cells mature within their respective progenitor zones and begin to exit the cell cycle, an abrupt transition occurs in the transcriptional profile of cells. The postmitotic cells from some progenitor domains (e.g., p2) become further diversified through intercellular signaling interactions (e.g., Notch-Delta), which leads to the generation of excitatory V2a and inhibitory V2b neurons from common ancestral progenitor cells. As neuron development progresses, the neurotransmitter properties of the cells emerge, and they express phenotypic markers, such as neurotransmitter biosynthetic enzymes (e.g., choline acetyltransferase [ChAT] or glutamic acid decarboxylase [GAD]) and vesicular transport proteins (e.g., vGluT2). Many progenitor domains tend to give rise to neurons with similar initial axonal growth trajectories; however, the pd1, p1, and p0 domains produce interneuron subtypes with diverse axonal projections, which are clear exceptions to this trend. The diversification of a single neuronal class is best exemplified by motor neurons.
Motor Neurons
Motor neurons are subdivided by their cell body positions within motor columns. Each motor column consists of multiple motor pools that innervate individual muscles. Generic postmitotic motor neurons become subdivided into the medial and hypaxial motor columns (MMC and HMC), which innervate the back (epaxial) and trunk (hypaxial) musculature, respectively. At limb levels, the medial and lateral portions of the lateral motor column (LMCm and LMCl) innervate the ventral and dorsal portions of the limb, respectively. Additional motor neurons form the autonomic nervous system as preganglionic (PGC) cholinergic neurons of sympathetic and parasympathetic targets. Pools of motor neurons innervating the same muscle can be defined by unique combinations of transcription factors (e.g., Nkx6 and Ets classes).
Locomotor Circuitry
The spinal interneurons and motor neurons comprise a central pattern-generating circuitry that is capable of producing rhythmic left-right and flexor-extensor alternation in isolated cords, called fictive locomotion. Molecular genetic studies have defined roles for several classes of interneurons found in the ventral cord. Mutant mice lacking contralat-erally projecting inhibitory Dbx1+ V0 class interneurons display a disorganized left-right alternation. Use of diphtheria toxin to ablate ipsilaterally projecting excitatory Chx10+ V2a cells also disturbs right-left alternation and, notably, at higher speeds, animals transition to a left-right synchronous gallop that is not seen in wild-type mice. Loss or inactivation of ipsilateral inhibitory En1+ V1 neurons results in a marked slowing of locomotion, whereas inactivation of contralaterally projecting excitatory Sim1+ V3 interneuron class disrupts the regularity of the rhythm. Inactivation of the Pitx2+ V0C class disrupts locomotion during swimming due to altered integration of sensory feedback. The ipsilaterally projecting glutamatergic Hb9+ Vx class is rhythmically active during locomotion. The Ptf1a+ dI4 class forms inhibitory presynaptic contacts on glutamatergic proprioceptive sensory neurons in the ventral spinal cord. The dI6 and dI3 interneuron classes make direct connections onto motor neurons; however, their roles have not been determined.
Abbreviations
- Ascl1
achaete-scute complex homolog 1 (Drosophila)
- BarH1
BarH-like homeobox
- Bhlhb5
basic-helix-loop-helix family, member e22
- Bmp2
bone morphogenetic protein 2
- Bmp4
bone morphogenetic protein 4
- Bmp5
bone morphogenetic protein 5
- Bmp6
bone morphogenetic protein 6
- Bmp7
bone morphogenetic protein 7
- Bmpr1a
bone morphogenetic protein receptor, type 1a
- Bmpr1b
bone morphogenetic protein receptor, type 1b
- Brn3a
POU domain, class 4, transcription factor 1
- Chx10
visual system homeobox
- Dbx1
developing brain homeobox
- Dbx2
developing brain homeobox
- Isl1
ISL1 transcription factor, LIM homeodomain
- Isl2
ISL2 transcription factor, LIM homeodomain
- Lbx1
ladybird homeobox homolog 1
- Lhx1
LIM homeobox protein 1
- Lhx2
LIM homeobox protein 2
- Lhx4
LIM homeobox protein 4
- Lhx5
LIM homeobox protein 5
- Lhx9
LIM homeobox protein 9
- Lmo4
LIM domain only 4
- Lmx1b
LIM homeobox transcription factor 1 beta
- Math1
atonal homolog 1 (Drosophila)
- Msx1
homeobox, msh-like 1
- Ngn1
neurogenin 1
- Ngn2
neurogenin 2
- Ngn3
neurogenin 3
- Notch
Notch gene homolog 1 (Drosophila)
- Nkx2.2
NK2 transcription factor related locus 2
- Nkx2.9
NK2 transcription factor related, locus 9
- Nkx6.1
NK6 homeobox 1
- Nkx6.2
NK6 homeobox 2
- Olig2
oligodendrocyte transcription factor 2
- Olig3
oligodendrocyte transcription factor 3
- Pax2
paired box gene 2
- Pax3
paired box gene 3
- Pax6
paired box gene 6
- Pax7
paired box gene 7
- PLCgamma
phospholipase C, gamma 1
- Ptf1a
pancreas-specific transcription factor 1a
- Pitx2
paired-like homeodomain transcription factor 2
- RA
retinoic acid
- Scl
T cell acute lymphocytic leukemia 1
- Shh
Sonic hedgehog
- Sim1
single-minded homolog 1
- Sox1
SRY box-containing gene 1
- Sox14
SRY box-containing gene 14
- Sox21
SRY box-containing 21
- Tlx1/3
T cell leukemia, homeobox 1/3
- Wt1
Wilms tumor homolog 1
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