Motor neurons with axial muscle projections specified by Wnt4/5 signaling

Abstract

Axial muscles are innervated by motor neurons of the median motor column (MMC). In contrast to the segmentally restricted motor columns that innervate limb, body wall, and neuronal targets, MMC neurons are generated along the entire length of the spinal cord. We show that the specification of MMC fate involves a dorsoventral signaling program mediated by three Wnt proteins (Wnt4, Wnt5a, and Wnt5b) expressed in and around the floor plate. These Wnts appear to establish a ventral(high) to dorsal(low) signaling gradient and promote MMC identity and connectivity by maintaining expression of the LIM homeodomain proteins Lhx3/4 in spinal motor neurons. Elevation of Wnt4/5 activity generates additional MMC neurons at the expense of other motor neuron columnar subtypes, whereas depletion of Wnt4/5 activity inhibits the production of MMC neurons. Thus, two dorsoventral signaling pathways, mediated by Shh and Wnt4/5, are required to establish an early binary divergence in motor neuron columnar identity.

Figure 1

Motor Column Organization in the Spinal Cord Motor columns in the spinal cord. Median Motor Column (MMC, red) neurons are present at all segmental levels of the spinal cord and innervate axial musculature. Segmentally restricted motor columns (S-MC) are confined to discrete segmental levels. LMC neurons (green) project to limb musculature and are present at brachial and lumbar levels, whereas at thoracic levels hypaxial motor column neurons (HMC, purple) innervate body wall muscles, and preganglionic autonomic motor neurons (PGCs, brown) innervate sympathetic ganglia.

Figure 2

MMC Fate as a Function of the Dorsoventral Position of Motor Neuron Generation (A–D) Lhx3 (red) and Isl1/2 (green) expression in thoracic spinal cord from Nkx2.2^+/− (A and B) and Nkx2.2^−/− (C and D) mice at e13.5. Isl1/2^on neurons in the ventral spinal cord are motor neurons (MNs); Lhx3^on, Isl1/2^on cells are MMC neurons; and Lhx3^on, Isl1/2^off neurons are V2a interneurons. Note the increase in MMC neuron number (yellow) in Nkx2.2 mutants. PGC neurons are located dorsolaterally. (E–H) In ovo electroporation of a constitutively active Smoothened receptor (Smo^W535L) in chick spinal cord is revealed by marker eGFP expression (E). (F–H) Lhx3 (red) and Isl1/2 (green) expression in transfected thoracic chick spinal cord (HH stage 29). PGC neurons are located dorsomedially in the chick. (I) Plots of motor neuron numbers per ventral 15 μm quadrant in Nkx2.2^+/− (open circles) and Nkx2.2^−/− embryos (closed circles); mean ± SEM (n = 5 embryos per genotype, Student's t test, ^∗∗p < 0.01). (J) Plot of motor neuron numbers for Smo^W535L-transfected embryos (mean ± SEM; n = 8 embryos, Student's t test, ^∗∗p < 0.01, open circles represent controls and closed circles Smo^W535L-transfected sides). (K) Summary of ectopic generation of motor neurons along the dorsoventral axis of the spinal cord in Nkx2.2^−/− mutant mouse and Smo^W535L chick embryos.

Figure 3

Expression of Wnt Genes in the Ventral Spinal Cord (A–I) Expression of Wnt4 (A–C), Wnt5a (D–F), and Wnt5b (G–I) in the ventral spinal cord of e9.5 and e10.5 mouse embryos and HH stage 19–20 chick embryos. Wnt4 is expressed in the floor plate, the p3 domain, and the dorsal spinal cord in the mouse (A and B) but not in the chick floor plate (C). Wnt5a is expressed broadly in the ventral spinal cord of e9.5 and e10.5 mouse embryos (D and E) but is largely restricted to the floor plate and p3 domain in chick embryos (F). Wnt5b is restricted to the floor plate in mouse and chick spinal cord (G–I). (J–L) Plots of Wnt4/5a/5b transcript expression level in the ventral spinal cord of e9.5 mouse (J), e10.5 mouse (K), and chick (L) embryos. (M–O) Wnt1 (M) expression in e10.0 mouse spinal cord and Wnt7a (J) and Wnt7b (K) expression in e9.5 mouse spinal cord. Wnt1 is expressed in the roof plate. Wnt7a and Wnt7b are present in the ventral spinal cord but are excluded from the floor plate and p3 domain. (P) Plots of Wnt7a and Wnt7b transcripts in the ventral spinal cord of e9.5 mouse embryos. In all plots, the x axis represents transcript levels and the y axis distance (μm) from the ventral midline to intermediate spinal cord (black arrow).

Figure 4

Misexpression of Wnt4/5 Genes Specifies MMC Columnar Identity Electroporation of stage 12–14 chick thoracic spinal cord with Wnt5a (A–C and J), Wnt4 (D–F and K) or Wnt1 (G–I and L). Tracer eGFP expression (A, D, and G) labels the transfected side of the spinal cord. Different combinations of Lhx3 (red) and Isl1/2 (green) expression mark MMC, HMC, and PGC neurons (B, E, and H). Thoracic misexpression of Wnt5a does not change total motor neuron number, but leads to an increase in the number of MMC neurons and to a decrease in HMC neurons (A–C and J). Wnt4 and Wnt5b (not shown) transfection produces effects similar to Wnt5a (D–F and K). Ectopic MMC neurons also express Lhx4 (C and F). Transfection of Wnt1 increases the total number of motor neurons, but not the number of MMC neurons (H). Note the aberrant migration of PGC neurons in thoracic segments of Wnt1-transfected embryos (H and I). Wnt4- and Wnt5a-induced ectopic MMC neurons also express Lhx4 (C and F). (J–L) Plots of motor neuron columnar subtype per ventral quadrant 15 μm section in Wnt5a (J), Wnt4 (K), and Wnt1 (L) transfected embryos, mean ± SEM (Student's t test, ^∗∗p < 0.01; ^∗p < 0.05; n = 20 embryos for Wnt5a, n = 16 embryos for Wnt4, and n = 10 embryos for Wnt1 transfection).

Figure 5

MMC Neurons Induced by Wnt5a Project to Axial Muscles (A) HRP labeling of MMC neurons after tracer injection into axial muscles in control or Wnt5a-transfected chick spinal cords. (B–I) eGFP expression marks the electroporated side of the spinal cord (B and F), Lhx3 and Isl1/2 expression (C and G), HRP labeling of Isl1/2^on neurons (D and H) HRP labeling of Lhx3^on neurons (E and I) in control and Wnt5a-electroporated thoracic spinal cord. There is an increase in the number of MMC neurons (G) and the number of HRP-labeled Lhx3^on Isl1/2^on motor neurons (H and I) in Wnt5a-transfected embryos. (J and K) Plots of MMC neurons (J) and HRP^on, Lhx3^on motor neurons (K) in controls (open circle) or Wnt5a-transfected embryos (closed circle). Circles with bars represent mean ± SEM (Student's t test, ^∗∗p < 0.01, n = 8 embryos).

Figure 6

Thoracic Motor Neuron Columnar Subtypes in Wnt4/5 Mutant Mice (A–E) Lhx3 and Isl1/2 expression in e13.5 thoracic spinal cord from wild-type and allelic combinations of Wnt4, Wnt5a, and Wnt5b mutant mice. Isl1/2^on ventral neurons are motor neurons, and Lhx3^on Isl1/2^on neurons (yellow) are MMC neurons. (F–I) Plots representing the total number of motor neurons (F), PGC neurons (G), MMC neurons (H), and HMC neurons (I) at thoracic levels in various mutant Wnt4, Wnt5a, and Wnt5b allelic combinations; mean ± SEM (ANOVA test, ^∗∗p < 0.01, ^∗p < 0.05, n = 3–6 embryos for different genotypes, except Wnt4^−/−; Wnt5a^−/−; Wnt5b^+/− [n = 1]).

Figure 7

Wnt4/5 Signaling and the Specification of MMC Identity (A) Fractional representation of MMC neurons in various experimental conditions, with quantification derived from our thoracic-level data. Wnt4, Wnt5a, and Wnt5b are distributed in a ventral^high to dorsal^low gradient in the ventral spinal cord (red triangle). Model shows a scenario in which cells located in more ventral regions of the pMN domain have a higher probability of acquiring MMC identity (red cells), whereas cells located at more dorsal regions of the pMN domain are more likely to acquire segmental column fates (S-MC, blue cells). This model is supported by the finding that in Nkx2.2^−/− mice all motor neurons generated from the p3 domain acquire MMC identity, whereas in Smo^W535L chick embryos, neurons generated at positions dorsal to the pMN domain acquire segmental columnar fates. Misexpression of Wnt4, Wnt5a, or Wnt5b increases the fractional allocation of MMC neurons, whereas their proportion is reduced in mice that have five mutated Wnt4, Wnt5a, or Wnt5b alleles. (B) Model for the relative contributions of Wnt4/5 and FGF/Hox signaling in the diversification of motor neuron columnar identities. Shh-induced motor neuron progenitors (gray) are exposed to competing signals. Graded Wnt4/5 signals along the DV axis (orange arrow) promote the maintenance of Lhx3 and acquisition of an MMC fate (red). FGF signaling along the AP axis induces differential Hox expression (blue line) so specifying LMC (green) and PGC (brown) fates at limb and thoracic levels, respectively. Some motor neurons at thoracic levels evade Wnt4/5 and FGF-Hox activity and progress to an HMC fate (purple). For details, see text and Dasen et al. (2008).