Fig. 2. BMP signaling regulates the expression of homeobox genes in the neural tube. (A-D) Expression of Msx genes is upregulated by BMP signaling. (A,B) Msx2 RNA expression. A stage 24 wild-type sample. (B) Expanded Msx2 expression following electroporation of activated BMPR-Ib (stage 23). (C,D) Msx1 RNA expression. (C) Stage 24 wild-type embryo. (D) Upregulation of Msx1 expression in an electroporated embryo expressing activated BMPR-Ib (stage 23). Expression is detected in the ventral neural tube and at higher levels within the endogenous expression domain. (E-H) Dbx protein expression is repressed by BMP signaling. (E,F) Dbx1 protein expression. (E) Stage 23 wild-type sample. (F) Cells expressing activated BMPR cease to express Dbx1 (stage 23, viral infection; arrowhead denotes region of BMP-driven repression). Inset shows viral infection in green; the absence of Dbx1 correlates with viral infection. (G,H) Dbx2 protein expression. (G) Stage 24 wild-type sample. (H) Repression of Dbx2 expression in a stage 23 sample expressing activated BMPR-Ib following viral infection. Arrowhead indicates a group of cells that no longer express Dbx2. (I-L) Differentiated cell types of the intermediate neural tube are altered in response to BMP signaling. (I,J) En1 protein expression, which marks the interneurons derived from the ventral-most Dbx2-expressing cells. (I) Wild-type expression in a stage 24 sample. (J) Arrowhead indicates that this population is greatly reduced in samples expressing activated BMPR (stage 24, electroporation). Inset shows that the absence of En1 expression correlates with extensive transfection (marked by GFP). (K,L) Evx protein expression, which marks the interneurons derived from Dbx1+, Dbx2+, Pax7– cells. (K) Wild-type expression in a stage 24 sample. (L) Arrowhead indicates that this population is greatly reduced in samples expressing activated BMPR (stage 24, electroporation). GFP fluorescence indicates extensive transfection of cells in the region that would normally express Evx.
