Fig. 5. BMP signaling regulates the generation of terminally differentiated dorsal interneurons. (A) Ngn1 RNA expression is activated at a threshold of BMP signaling. Ngn1 is activated by low levels of BMP signaling. A construct that weakly expresses activated BMPR-Ib (EGFP vector) results in an increase in Ngn1-expressing cells, apparently by activating it in cells ventral to its normal domain (arrowhead, stage 24). (B,C) LH2A/B-expressing cells are generated throughout the dorsal neural tube in response to high levels of BMP signaling. (B) In stage 23 wild-type samples, LH2A/B-expressing cells are generated by those cells that express Cath1 and migrate ventrally. (C) In response to high level BMP signaling, LH2A/B⁺ cells are generated throughout the dorsal neural tube (arrowhead, stage 24). Inset shows co-localization of GFP and LH2A/B expression on the transfected side of the neural tube. (D,E) High levels of BMP signaling block the development of dorsal Lim1/2⁺ cells. (D) In wild-type samples, Lim1/2⁺ cells are produced in two locations within the dorsal neural tube (bars; stage 23). (E) Expression of activated BMPR reduces or eliminates the generation of the two dorsal populations of Lim1/2-expressing cells (arrowhead denotes normal region of their generation, stage 23). Inset shows GFP expression, indicating transfected cells. Note that suppression of Lim1/2 expression is specific to dorsal cells, as intermediate expression continues in transfected cells. (F-H) Isl1-expressing cells are formed within a narrow range of BMP signaling activity. (F) Samples with slight increases in BMP signaling (generated with the same vector as in A, EGFP-activated BMPR-Ib) generate Isl1-expressing cells over a broader region of the dorsal neural tube (bracketed; bar indicates endogenous domain, late stage 24). Inset shows GFP label of transfected cells. Note that Isl1 expansion is restricted to the region near its normal expression domain, despite extensive transfection. (G) In stage 24 wild-type samples, Isl1 expression marks a small population of dorsal interneurons (bar) as well as the DRG and motoneurons.(H) High levels of BMP signaling, however, abolish the generation of Isl1⁺ interneurons (arrowhead, stage24). Inset shows that the absence of Isl1 is correlated with the presence of transfected cells marked by GFP.