Fig. 4. Expression of activated BMPR alters the expression of bHLH proteins in the dorsal neural tube. (A,B) Cath1 expression expands ventrally in response to BMP signaling. (A) Wild-type expression of Cath1 protein occurs in cells immediately ventral to the roofplate (stage 23 sample). (B) Expression of Cath1 expands dramatically in response to expression of activated BMPR (stage 24, viral infection). Note that the expanded expression remains restricted to dorsal neural tissue. Inset shows viral antigen expression, indicating infected cells; expanded Cath1 correlates with areas of extensive viral infection. (C,D) Cash1 expression is repressed by BMP signaling. (C) Wild-type expression of Cash1 protein includes a broad band of cells in the dorsal neural tube (stage 24). (D) The Cash1 domain is reduced in samples expressing activated BMPR (stage 23 viral infection). (E,F) Dorsal Ngn2 expression is reduced by BMP signaling. (E) Wild-type expression of Ngn2 RNA is found in ventricular and subventricular cells in the dorsal half of the neural tube, as well as a large population of ventral cells (stage 24; arrowhead indicates dorsal expression domain). (F) Expression of Ngn2 in most dorsal cells is extinguished by the expression of activated BMPR (stage 24 electroporation); ventral expression remains unaffected. (G,H) Dorsal expression of Ngn1 is repressed by BMP signaling. (G) At stage 24, Ngn1 RNA is expressed by cells of the dorsal root ganglia (DRG), proliferating ventral cells and a narrow band of cells in the dorsal neural tube (arrowhead). (H) Ngn1 expression in the dorsal neural tube is absent in samples expressing activated BMPR (stage 24 electroporation), while ventral and DRG expression remains normal.