Ems and Nkx6 are central regulators in dorsoventral patterning of the Drosophila brain

Abstract

In central nervous system development, the identity of neural stem cells (neuroblasts) critically depends on the precise spatial patterning of the neuroectoderm in the dorsoventral (DV) axis. Here, we uncover a novel gene regulatory network underlying DV patterning in the Drosophila brain, and show that the cephalic gap gene empty spiracles (ems) and the Nk6 homeobox gene (Nkx6) encode key regulators. The regulatory network implicates novel interactions between these and the evolutionarily conserved homeobox genes ventral nervous system defective (vnd), intermediate neuroblasts defective (ind) and muscle segment homeobox (msh). We show that Msh cross-repressively interacts with Nkx6 to sustain the boundary between dorsal and intermediate neuroectoderm in the tritocerebrum (TC) and deutocerebrum (DC), and that Vnd positively regulates Nkx6 by suppressing Msh. Remarkably, Ems is required to activate Nkx6, ind and msh in the TC and DC, whereas later Nkx6 and Ind act together to repress ems in the intermediate DC. Furthermore, the initially overlapping expression of Ems and Vnd in the ventral/intermediate TC and DC resolves into complementary expression patterns due to cross-repressive interaction. These results indicate that the anteroposterior patterning gene ems controls the expression of DV genes, and vice versa. In addition, in contrast to regulation in the ventral nerve cord, cross-inhibition between homeodomain factors (between Ems and Vnd, and between Nkx6 and Msh) is essential for the establishment and maintenance of discrete DV gene expression domains in the Drosophila brain. This resembles the mutually repressive relationship between pairs of homeodomain proteins that pattern the vertebrate neural tube in the DV axis.KEY WORDS: Brain development, Neuroectodermal regionalization, Stem cell specification, Drosophila, ems/Emx, Nkx6 (HGTX), vnd/Nkx2, msh (Drop)/Msx, ind/Gsx

Genetic regulation and function of epidermal growth factor receptor signalling in patterning of the embryonicDrosophilabrain

“…To test whether EGFR controls DV patterning in the early brain, we first compared the activation pattern of EGFR with the known expression patterns of the DV genes vnd , ind , Nkx6 and msh [52,53]. We visualized EGFR activity in the brain NE by using an antibody against double-phosphorylated (activated) MAPK [27].…”

“…At stage 6 (which is slightly prior to the expression of the segmental marker En in the brain NE), tentative boundaries between presumptive brain neuromeres were estimated with regard to the distance from the cephalic furrow (CF; white arrow in ( a–d )) in AP axis and the AP extent of DV gene expression domains (i.e. ind in ( c ) and Nkx6 in ( d ); see also [52,53]). Dashed lines in white indicate the ventral midline.…”

“…In previous reports, we showed that the regionalized expression of DV genes exhibits neuromere-specific differences in the NE and neuroblasts of the embryonic brain [50,51]. We then uncovered a genetic network in which evolutionarily conserved factors encoded by DV genes ( vnd , ind , msh , Nkx6 ) and AP patterning genes ( empty spiracles , engrailed ) closely interact to properly pattern the NE and specify neuroblast identity along the DV axis of the brain [52,53]. …”

Genomic Control Processes in Adult Body Part Formation

“…This is not to be confused with a "bi-stable switch" since there is no bi-stability, and the system is, like the rest of early development, entirely determinate: in normal development the A repressor is always exclusively expressed in subdomain A and the B repressor is always exclusively expressed in subdomain B. Figure 4.5(B-E) show four examples, of many, in which the boundaries between regulatory state domains are violated by the across-boundary regulatory state in genetic perturbations where one of the genes of a mutually repressing pair is mutated or ectopically expressed (Seibert et al, 2009;Seibert and Urbach, 2010). In Fig.…”

“…(A4), Same displays, with definitive territories at early stage 11; FG, foregut, represented by lobes at upper right of diagrams here and in (A3). (B), Mutual repression between ems and vnd, in situ hybridization for vnd mRNA and immunocytology for Ems protein (fromSeibert et al (2009)). (B1), Simultaneous expression of both genes at stage 5 in neuroectoderm of future ventral/intermediate DC and TC, when ems provides positive inputs into several brain regulatory genes (see text).…”