Retinoid signaling and neurogenin2 function are coupled for the specification of spinal motor neurons through a chromatin modifier CBP

Abstract

Extracellular signals and cell-intrinsic transcription factors cooperatively instruct generation of diverse neurons. However, little is known about how neural progenitors integrate both cues and orchestrate chromatin changes for neuronal specification. Here, we report that extrinsic signal retinoic acid (RA) and intrinsic transcription factor Neurogenin2 (Ngn2) collaboratively trigger transcriptionally active chromatin in spinal motor neuron genes during development. Retinoic acid receptor (RAR) binds Ngn2 and is thereby recruited to motor neuron genes targeted by Ngn2. RA then facilitates the recruitment of a histone acetyltransferase CBP to the Ngn2/RAR-complex, markedly inducing histone H3/H4-acetylation. Correspondingly, timely inactivation of CBP and its paralog p300 results in profound defects in motor neuron specification and motor axonal projection, accompanied by significantly reduced histone H3-acetylation of the motor neuron enhancer. Our study uncovers the mechanism by which extrinsic RA-signal and intrinsic transcription factor Ngn2 cooperate for cell fate specification through their synergistic activity to trigger transcriptionally active chromatin.

Figure 1. RA-signal stimulates Ngn2 activity through the formation of the Ngn2/RAR-complex

(A) RA enhances the transcriptional activity of Ngn2, but not Ngn2-AQ, in E-box:LUC reporter in P19 cells. Ngn2-AQ is a Ngn2 mutant that no longer binds E-box. RAR^ΔAF2, a RAR mutant that is unable to interact with AF2-dependent coactivators, inhibits the stimulating effect of RA on E-box:LUC. (B) Ngn2 binds RAR in a RA-independent manner in HEK293 cells, as determined by immunoprecipitation (IP) of lysates of HEK293 cells expressing Flag-Ngn2 with anti-Flag antibody followed by immunoblotting with anti-RAR antibody. (C) CoIP assays in HEK293 cells transfected with Flag-Ngn2 and either HA-RAR or HA-RAR^ΔAF2. Both RAR and RAR^ΔAF2 interact with Ngn2 in a RA-independent manner. (D) P19 cells transfected with vector or Ngn2-ires-GFP were treated with vehicle or RA, and analyzed for neuronal differentiation using immunostaining with anti-TuJ antibody. Arrows indicate neurite-like processes. (E) % quantification of TuJ⁺-neurons among GFP⁺-transfected P19 cells under indicated condition. (F, G) Immunohistochemical analysis of neuronal differentiation (TuJ⁺-cells) in HH stage 20 chick embryos electroporated with the indicated constructs on the bottom. RAR^ΔAF2 compromises precocious neuronal differentiation triggered by Ngn2 in the medial zone of the chick neural tube. GFP⁺-cells mark electroporated cells. (H) Quantification of GFP (green) and TuJ (red) fluorescence intensity in the neural tube. The X-axis indicates the most medial to lateral sides of the neural tube. Coexpression of Ngn2-ires-GFP and LacZ leads to the increase in TuJ staining concomitant with GFP expression in the medial zone of the neural tube (upper panel), whereas GFP expression does not correlate with TuJ staining in the medial zone of the neural tube electroporated with Ngn2-ires-GFP and RAR^ΔAF2 (bottom panel). (A, E) The error bars represent the standard deviation.

Figure 2. CBP is a key effector for RA-signal to promote Ngn2 function

(A) RA enhances in vivo association between Ngn2 and CBP in CoIP experiments, as tested with HEK293 cells transfected with HA-Ngn2. (B) CoIP assays using IP with anti-HA antibody followed by immunoblotting with anti-CBP antibody in HEK293 cells transfected with HA-Ngn2 and CBP along with either RAR or RAR^ΔAF2. RAR expression further strengthens the association of Ngn2 and CBP, while RAR^ΔAF2 attenuates this interaction. (C, D) Quantitative-RT-PCR for neurofilament M (NF-M) (C) and NeuroD (D) in P19 cells treated as indicated. (E) Immunohistochemical analysis of neuronal differentiation (TuJ⁺-cells) in HH stage 22 chick embryos electroporated with the indicated constructs on the left. Cells coexpressing Ngn2 and E1A fail to differentiate to TuJ⁺-neurons, suggesting that Ngn2-mediated neurogenesis is suppressed by E1A. In contrast, E1AΔN does not block neurogenesis in the medial zone of the chick neural tube (data not shown). (F) Quantitative analysis of neuronal differentiation in the chick neural tube. (C, D, F) The error bars represent the standard deviation.

Figure 3. RA-signaling promotes the specification of a motor neuron fate by Ngn2

(A) ChIP assays using IP with anti-RAR antibody in P19 cells transfected with Ngn2 or Ngn2-AQ. DNA-binding activity of Ngn2 is required to recruit RAR to MNe in a RA-independent manner, as shown by occupancy of MNe by RAR in the presence of Ngn2 but not Ngn2-AQ. β-RARE is a cognate DNA response element of RAR. (B) ChIP assays in P19 cells transfected with HA-RAR^ΔAF2 and Ngn2 or Ngn2-AQ. RAR^ΔAF2 is recruited to MNe by Ngn2. (C) RA synergizes with Ngn2 and Isl1/Lhx3 in activating MNe:LUC reporter in P19 cells. (D-F) RA stimulates motor neuron differentiation in P19 cells transfected with Ngn2, Isl1 and Lhx3, as monitored by immunostaining with anti-Hb9 antibody (D, E) and measuring Hb9 mRNA levels in quantitative RT-PCR (F). GFP⁺-cells mark transfected cells (D). (C, E, F) The error bars represent the standard deviation. (E) Asterisk, p<0.001 in the two-tailed t-test.

Figure 4. Transactivation, but not RARE-binding, by RA-bound RAR is required for Ngn2 to specify motor neurons

(A) Schematic representation of RARα CoIP assays using IP with anti-HA antibody followed by immunoblotting with anti-Flag antibody in HEK293 cells transfected with Flag-Ngn2 and either HA-RAR^DBDmt or HA-RAR^DBDmtΔAF2. Both RAR^DBDmt and RAR^DBDmtΔAF2 associate with Ngn2 independently of RA. (C-Q) Immunohistochemical analysis of ectopic motor neuron induction or formation of endogenous motor neurons (ventral motor neurons below dotted lines in E, H, K, N, Q) in HH stage 25 chick embryos electroporated with the indicated constructs on the top. (R, S) Quantitative analysis of ectopic motor neuron (MN) generation in the dorsal spinal cord (R) or formation of endogenous motor neurons in the ventral spinal cord (S). RAR^ΔAF2 and RAR^DBDmtΔAF2, but not RAR wild-type or RAR^DBDmt, block the specification of motor neuron cell-type. Asterisk, p<0.001 in the two-tailed t-test. The error bars represent the standard deviation.

Figure 5. CBP is required for proper motor neuron development

(A) RA facilitates the recruitment of CBP to MNe in P19 cells expressing Ngn2, as shown by ChIP using anti-CBP antibody. (B-U) Immunostaining of motor neuron markers Hb9, Isl1, Isl2, and VAChT and a V2-interneuron marker Ch×10 in E12.0 CBP mutants and E11.5 CBP/p300-compound mutants. The ventral quadrant of the spinal cord is shown. Ectopically emigrating motor neurons outside the spinal cord are marked by parentheses. These emigrating cells coexpress Isl2 and VAChT, as indicated by yellow arrows. (V) Quantification of Hb9⁺-motor neurons in both inside and outside the spinal cord of CBP/p300 mutants at cervical levels. The number of motor neurons of each genotype over their control littermate is shown in %. (W) Quantification of ectopically emigrating Hb9⁺-motor neurons outside the spinal cord in CBP/p300 mutants at thoracic levels. The number of emigrating motor neurons over the total number of motor neurons is shown in %. (V, W) One asterisk, p<0.01; two asterisks, p<0.001; three asterisks, p<0.0001 in the two-tailed t-test. The error bars represent the standard deviation. Scale bars, 100 μm (B-K), 50 μm (L-U).

Figure 6. Motor axonal trajectory is profoundly disrupted in CBP/p300 mutants

Immunostaining analyses of motor neuron markers Isl2 and VAChT in E12.0 CBP mutants (A-F) and E11.5 p300- and CBP/p300-compound mutants (G-R). Spinal cord and dorsal root ganglion (DRG) are outlined by dotted lines. Arrows indicate erroneous projection of VAChT⁺ motor axons toward dorsal root entry zone (DREZ), roof plate, midline and within DRG.

Figure 7. Timely inactivation of CBP/p300 during motor neuron specification impairs motor neuron development

(A-L) Immunohistochemical analyses in E11.5 CBP^f/+;p300^f/f;Isl1-Cre^- (A-F) and CBP^f/+;p300^f/f;Isl1-Cre⁺ (G-L) embryos. Ectopically emigrating motor neurons outside the spinal cord (parenthesis), widened motor exit points (yellow arrows), and erroneous projection of VAChT⁺ motor axons toward DREZ, roof plate and within DRG (pink arrows) are marked. (M, N) Quantification of Hb9⁺-motor neurons in the spinal cord (M) and ectopically emigrating Hb9⁺-motor neurons outside the spinal cord (N) in 12 μm sections at thoracic levels. Asterisk, p<0.001 in the two-tailed t-test. The error bars represent the standard deviation.

Figure 8. Transcriptionally active chromatin is established by RA and CBP in MNe

(A) The histone modifications in MNe upon RA treatment were analyzed by ChIP assays in P19 cells expressing Ngn2. RA facilitates histone H3/H4-acetylation and H3-lysine-4-trimethylation (H3K4me3), while suppressing H3-lysine-9-diemthylation (H3K9me2), in MNe. (B, C) ChIP assays using the spinal cord dissected from mutant embryos of genotypes shown in the boxes. Histone H3-acetylation (B) and H3K4me3 (C) in MNe are impaired in CBP-inactivated E12.5 embryonic spinal cord. (D) The working model. Ngn2 and RAR form a complex in pMN progenitors. The extrinsic signal RA binds RAR and facilitates the association of a chromatin modifier CBP with the Ngn2/RAR-complex. Assembly of the Ngn2/RAR/CBP-complex on Ngn2-target motor neuron enhancers triggers their transcriptionally active open chromatin structure marked by H3/4-acetylation and results in subsequent motor neuron gene expressions, leading to the differentiation to motor neurons.