DLX-1, DLX-2, and DLX-5 expression define distinct stages of basal forebrain differentiation

The homeobox genes in the Dlx family are required for differentiation of ba sal forebrain neurons and craniofacial morphogenesis. Herein, we studied th e expression of Dlx-1, Dlx-2, and Dlx-5 RNA and protein in the mouse forebr ain from embryonic day 10.5 (E10.5) to E12.5. We provide evidence that Dlx- 2 is expressed before Dlx-1, which is expressed before Dlx-5. We also demon strate that these genes are expressed in the same cells, which may explain why single mutants of the DL genes have mild phenotypes. The DLX proteins a re localized primarily to the nucleus, although DLX-5 also can be found in the cytoplasm. During development, the fraction of Dir-positive cells incre ases in the ventricular zone. Analysis of the distribution of DLX-1 and DLX -2 in M-phase cells suggests that these proteins are distributed symmetrica lly to daughter cells during mitosis. We propose that DLX-negative cells in the ventricular zone are specified progressively to become DLX-2-expressin g cells during neurogenesis; as these cells differentiate, they go on to ex press DLX-1, DLX-5, and DLX-6. This process appears to be largely the same in all regions of the forebrain that express the DL genes. In the basal tel encephalon, these DLX-positive cells differentiate into projection neurons of the striatum and pallidum as well as interneurons, some of which migrate to the cerebral cortex and the olfactory bulb. (C) 1999 Wiley-Liss, Inc.