RESUMEN
During animal development, the Hedgehog (Hh) signal transduction pathway plays critical roles in cell fate determination and tissue patterning. In humans, aberrant Hh signaling has been linked to several genetic disorders and cancers. Binding of Hh to its receptor initiates a signaling cascade, which ultimately results in the activation of the Gli/Ci transcription factors. Suppressor of fused (Su(fu)) is a Gli/Ci-interacting protein, which acts as a negative regulator of Hh signaling in Drosophila and vertebrates. Su(fu) is also implicated as a tumor suppressor as its mutations have been found in medulloblastoma and prostate cancer. Su(fu) is thought to act by preventing the nuclear accumulation of Gli/Ci, however, mechanistic insight into its mode of action has remained elusive. We demonstrate here that Su(fu) prevents the nuclear accumulation of Gli1 and Gli2 through multiple mechanisms. While Su(fu) itself is not subject to CRM1-dependent regulation, Su(fu) sequesters Gli1 in the cytoplasm mostly through a mechanism that depends on the activity of the nuclear export protein CRM1. In contrast, CRM1-mediated export is not required for Su(fu) to sequester Gli2. Furthermore, we show that the N-terminus of Su(fu) is sufficient for Gli inactivation in the absence of cytoplasmic sequestration. Together, these observations reveal that Su(fu) regulates the activity of Gli1 and Gli2 through distinct cytoplasmic and nuclear mechanisms.
Asunto(s)
Núcleo Celular/metabolismo , Factores de Transcripción de Tipo Kruppel/metabolismo , Proteínas Oncogénicas/metabolismo , Proteínas Represoras/fisiología , Factores de Transcripción/metabolismo , Transporte Activo de Núcleo Celular , Secuencia de Aminoácidos , Animales , Regulación de la Expresión Génica , Genes Supresores , Carioferinas/metabolismo , Carioferinas/fisiología , Ratones , Mutación Puntual , Receptores Citoplasmáticos y Nucleares/metabolismo , Receptores Citoplasmáticos y Nucleares/fisiología , Proteínas Represoras/metabolismo , Transactivadores , Activación Transcripcional , Transfección , Proteína con Dedos de Zinc GLI1 , Proteína Gli2 con Dedos de Zinc , Proteína Exportina 1RESUMEN
In the mouse retina, at least ten distinct types of bipolar interneurons are involved in the transmission of visual signals from photoreceptors to ganglion cells. How bipolar interneuron diversity is generated during retinal development is poorly understood. Here, we show that Irx5, a member of the Iroquois homeobox gene family, is expressed in developing bipolar cells starting at postnatal day 5 and is localized to a subset of cone bipolar cells in the mature mouse retina. In Irx5-deficient mice, defects were observed in the expression of some, but not all, immunohistological markers that define mature Type 2 and Type 3 OFF cone bipolar cells, indicating a role for Irx5 in bipolar cell differentiation. The differentiation of these two bipolar cell types has previously been shown to require the homeodomain-CVC transcription factor, Vsx1. However, the defects observed in Irx5-deficient retinas do not coincide with a reduction of Vsx1 expression, and conversely, the expression of Irx5 in cone bipolar cells does not require the presence of a functional Vsx1 allele. These results indicate that there are at least two distinct genetic pathways (Irx5-dependent and Vsx1-dependent) regulating the development of Type 2 and Type 3 cone bipolar cells.
Asunto(s)
Diferenciación Celular/genética , Genes Homeobox/fisiología , Proteínas de Homeodominio/fisiología , Retina/embriología , Células Fotorreceptoras Retinianas Conos/citología , Células Fotorreceptoras Retinianas Conos/fisiología , Factores de Transcripción/fisiología , Animales , Diferenciación Celular/fisiología , Proteínas del Ojo/fisiología , Proteínas de Homeodominio/genética , Ratones , Ratones Noqueados , Neuroglía/fisiología , Factores de Transcripción/genéticaRESUMEN
The Iroquois homeobox (Irx) genes have been implicated in the specification and patterning of several organs in Drosophila and several vertebrate species. Misexpression studies of chick, Xenopus, and zebra fish embryos have demonstrated that Irx genes are involved in the specification of the midbrain-hindbrain boundary. All six murine Irx genes are expressed in the developing heart, suggesting that they might possess distinct functions during heart development, and a role for Irx4 in normal heart development has been recently demonstrated by gene-targeting experiments. Here we describe the generation and phenotypic analysis of an Irx2-deficient mouse strain. By targeted insertion of a lacZ reporter gene into the Irx2 locus, we show that lacZ expression reproduces most of the endogenous Irx2 expression pattern. Despite the dynamic expression of Irx2 in the developing heart, nervous system, and other organs, Irx2-deficient mice are viable, are fertile, and appear to be normal. Although chick Irx2 has been implicated in the development of the midbrain-hindbrain region, we show that Irx2-deficient mice develop a normal midbrain-hindbrain boundary. Furthermore, Irx2-deficient mice have normal cardiac morphology and function. Functional compensation by other Irx genes might account for the absence of a phenotype in Irx2-deficient mice. Further studies of mutant mice of other Irx genes as well as compound mutant mice will be necessary to uncover the functional roles of these evolutionarily conserved transcriptional regulators in development and disease.