RESUMO
The Hedgehog (Hh) signal transduction pathway is essential for the development and patterning of numerous organ systems, and has important roles in a variety of human cancers. Genetic screens for mouse embryonic patterning mutants first showed a connection between mammalian Hh signaling and intraflagellar transport (IFT), a process required for construction of the primary cilium, a small cellular projection found on most vertebrate cells. Additional genetic and cell biological studies have provided very strong evidence that mammalian Hh signaling depends on the primary cilium. Here, we review the evidence that defines the integral roles that IFT proteins and cilia play in the regulation of the Hh signal transduction pathway in vertebrates. We discuss the mechanisms that control localization of Hh pathway proteins to the cilium, focusing on the transmembrane protein Smoothened (Smo), which moves into the cilium in response to Hh ligand. The phenotypes caused by loss of cilia-associated proteins are complex, which suggests that cilia and IFT play active roles in mediating Hh signaling rather than serving simply as a compartment in which pathway components are concentrated. Hh signaling in Drosophila does not depend on cilia, but there appear to be ancient links between cilia and components of the Hh pathway that may reveal how this fundamental difference between the Drosophila and mammalian Hh pathways arose in evolution.
Assuntos
Cílios/metabolismo , Proteínas Hedgehog/metabolismo , Transdução de Sinais/fisiologia , Animais , Padronização Corporal , Cílios/ultraestrutura , Proteínas Hedgehog/genética , Humanos , Camundongos , Mutação , Receptores Acoplados a Proteínas G/metabolismoRESUMO
In the ventral telencephalon, the medial ganglionic eminence (MGE) is a major source of cortical interneurons. Expression of the transcription factor NKX2.1 in the MGE is required for the specification of two major subgroups of cortical interneurons - those that express parvalbumin (PV) or somatostatin (SST) - but direct targets of NKX2.1 remain to be established. We find that electroporation of Nkx2.1 cDNA into the ventral telencephalon of slice cultures from Nkx2.1-/- mouse embryos, followed by transplantation into neonatal cortex to permit postnatal analysis of their fate, rescues the loss of PV- and SST-expressing cells. The LIM-homeobox gene Lhx6 is induced by this rescue experiment, and gain- and loss-of-function studies suggest that Lhx6 is necessary and sufficient to rescue these and other interneuron phenotypes in cells transplanted from Nkx2.1-/- slices. Finally, NKX2.1 protein binds a highly conserved sequence in the Lhx6 promoter, and this sequence appears to mediate the direct activation of Lhx6 by NKX2.1. The slice transfection and transplantation methods employed here are beginning to uncover embryonic mechanisms for specifying neuronal fates that only become definable postnatally.
Assuntos
Córtex Cerebral/embriologia , Córtex Cerebral/metabolismo , Proteínas de Homeodomínio/metabolismo , Interneurônios/citologia , Interneurônios/metabolismo , Proteínas do Tecido Nervoso/metabolismo , Proteínas Nucleares/metabolismo , Fatores de Transcrição/metabolismo , Animais , Sequência de Bases , Transplante de Tecido Encefálico , Diferenciação Celular , Córtex Cerebral/citologia , Primers do DNA/genética , Técnicas de Cultura Embrionária , Feminino , Regulação da Expressão Gênica no Desenvolvimento , Proteínas de Homeodomínio/antagonistas & inibidores , Proteínas de Homeodomínio/genética , Proteínas com Homeodomínio LIM , Eminência Mediana/citologia , Eminência Mediana/embriologia , Eminência Mediana/metabolismo , Camundongos , Camundongos Knockout , Camundongos Transgênicos , Proteínas do Tecido Nervoso/antagonistas & inibidores , Proteínas do Tecido Nervoso/genética , Proteínas Nucleares/deficiência , Proteínas Nucleares/genética , Parvalbuminas/genética , Fenótipo , Gravidez , Regiões Promotoras Genéticas , Interferência de RNA , Somatostatina/genética , Fator Nuclear 1 de Tireoide , Fatores de Transcrição/deficiência , Fatores de Transcrição/genética , TransfecçãoRESUMO
Mutations in human doublecortin (DCX) and knockdown of Dcx in rodents cause radial migration defects in the embryonic cerebral cortex. However, the brain phenotype of Dcx knockout mice is largely normal suggesting that Dcx is not necessary for most migration events. Adult subventricular zone (SVZ) cells migrate tangentially in the rostral migratory stream to the olfactory bulbs. Dcx is expressed in the SVZ but it is unknown if it is necessary for migration. We show that Dcx RNAi reduced SVZ cell migration in vitro, both cell autonomously and non-cell autonomously. Overexpression of Dcx rescued migration after knockdown, but did not increase migration by itself. Thus, Dcx is necessary not only for embryonic radial migration but also migration of adult SVZ cells.
