RESUMEN
The mammalian kidney is composed of thousands of individual epithelial tubules known as nephrons. Deficits in nephron number are associated with myriad diseases ranging from complete organ failure to congenital hypertension. A balance between differentiation and maintenance of a mesenchymal progenitor cell population determines the final number of nephrons. How this balance is struck is poorly understood. Previous studies have suggested that Wnt9b/ß-catenin signaling induced differentiation (mesenchymal-to-epithelial transition) in a subset of the progenitors but needed to be repressed in the remaining progenitors to keep them in the undifferentiated state. Here, we report that Wnt9b/ß-catenin signaling is active in the progenitors and is required for their renewal/proliferation. Using a combination of approaches, we have revealed a mechanism through which cells receiving the same Wnt9b/ß-catenin signal can respond in distinct ways (proliferate versus differentiate) depending on the cellular environment in which the signal is received. Interpretation of the signal is dependent, at least in part, on the activity of the transcription factor Six2. Six2-positive cells that receive the Wnt9b signal are maintained as progenitors whereas cells with reduced levels of Six2 are induced to differentiate by Wnt9b. Using this simple mechanism, the kidney is able to balance progenitor cell expansion and differentiation insuring proper nephron endowment. These findings provide novel insights into the molecular mechanisms that regulate progenitor cell differentiation during normal and pathological conditions.
Asunto(s)
Riñón/embriología , Nefronas/embriología , Transducción de Señal/fisiología , Células Madre/metabolismo , Proteínas Wnt/metabolismo , Animales , Diferenciación Celular , Proliferación Celular , Inmunoprecipitación de Cromatina , Proteínas de Homeodominio/genética , Proteínas de Homeodominio/metabolismo , Inmunohistoquímica , Hibridación in Situ , Riñón/citología , Riñón/metabolismo , Ratones , Nefronas/citología , Nefronas/metabolismo , Organogénesis/fisiología , Células Madre/citología , Factores de Transcripción/genética , Factores de Transcripción/metabolismo , Proteínas Wnt/genética , beta Catenina/genética , beta Catenina/metabolismoRESUMEN
The cranial base is essential for integrated craniofacial development and growth. It develops as a cartilaginous template that is replaced by bone through the process of endochondral ossification. Here, we describe a novel and specific role for the homeoprotein Six2 in the growth and elongation of the cranial base. Six2-null newborn mice display premature fusion of the bones in the cranial base. Chondrocyte differentiation is abnormal in the Six2-null cranial base, with reduced proliferation and increased terminal differentiation. Gain-of-function experiments indicate that Six2 promotes cartilage development and growth in other body areas and appears therefore to control general regulators of chondrocyte differentiation. Our data indicate that the main factors restricting Six2 function to the cranial base are tissue-specific transcription of the gene and compensatory effects of other Six family members. The comparable expression during human embryogenesis and the high protein conservation from mouse to human implicate SIX2 loss-of-function as a potential congenital cause of anterior cranial base defects in humans.
Asunto(s)
Base del Cráneo/crecimiento & desarrollo , Animales , Animales Recién Nacidos , Huesos , Cartílago/crecimiento & desarrollo , Cartílago/metabolismo , Diferenciación Celular , Condrogénesis , Proteínas de Homeodominio/metabolismo , Humanos , Ratones , Ratones Noqueados , Ratones Transgénicos , Proteínas del Tejido Nervioso , Osteogénesis/fisiología , Proteínas/metabolismoRESUMEN
The functional activity of Six2, a member of the so/Six family of homeodomain-containing transcription factors, is required during mammalian kidney organogenesis. We have now determined that Six2 activity is also necessary for the formation of the pyloric sphincter, the functional gate at the stomach-duodenum junction that inhibits duodenogastric reflux. Our data reveal that several genes known to be important for pyloric sphincter formation in the chick (e.g., Bmp4, Bmpr1b, Nkx2.5, Sox9, and Gremlin) also appear to be required for the formation of this structure in mammals. Thus, we propose that Six2 activity regulates this gene network during the genesis of the pyloric sphincter in the mouse.
Asunto(s)
Proteínas Fetales/fisiología , Redes Reguladoras de Genes/fisiología , Proteínas de Homeodominio/fisiología , Píloro/embriología , Factores de Transcripción/fisiología , Actinas/biosíntesis , Animales , Proteína Morfogenética Ósea 4/biosíntesis , Proteína Morfogenética Ósea 4/genética , Proteína Morfogenética Ósea 4/fisiología , Receptores de Proteínas Morfogenéticas Óseas de Tipo 1/biosíntesis , Receptores de Proteínas Morfogenéticas Óseas de Tipo 1/genética , Receptores de Proteínas Morfogenéticas Óseas de Tipo 1/fisiología , Citocinas , Reflujo Duodenogástrico/embriología , Proteínas Fetales/biosíntesis , Proteínas Fetales/genética , Mucosa Gástrica/embriología , Mucosa Gástrica/metabolismo , Regulación del Desarrollo de la Expresión Génica , Redes Reguladoras de Genes/genética , Proteína Homeótica Nkx-2.5 , Proteínas de Homeodominio/antagonistas & inhibidores , Proteínas de Homeodominio/biosíntesis , Proteínas de Homeodominio/genética , Péptidos y Proteínas de Señalización Intercelular/biosíntesis , Péptidos y Proteínas de Señalización Intercelular/genética , Péptidos y Proteínas de Señalización Intercelular/fisiología , Mesodermo/metabolismo , Ratones , Músculo Liso/metabolismo , Organogénesis , Píloro/anomalías , Factor de Transcripción SOX9/biosíntesis , Factor de Transcripción SOX9/genética , Factor de Transcripción SOX9/fisiología , Estómago/embriología , Factores de Transcripción/antagonistas & inhibidores , Factores de Transcripción/biosíntesis , Factores de Transcripción/deficiencia , Factores de Transcripción/genéticaRESUMEN
Nephrons, the basic functional units of the kidney, are generated repetitively during kidney organogenesis from a mesenchymal progenitor population. Which cells within this pool give rise to nephrons and how multiple nephron lineages form during this protracted developmental process are unclear. We demonstrate that the Six2-expressing cap mesenchyme represents a multipotent nephron progenitor population. Six2-expressing cells give rise to all cell types of the main body of the nephron during all stages of nephrogenesis. Pulse labeling of Six2-expressing nephron progenitors at the onset of kidney development suggests that the Six2-expressing population is maintained by self-renewal. Clonal analysis indicates that at least some Six2-expressing cells are multipotent, contributing to multiple domains of the nephron. Furthermore, Six2 functions cell autonomously to maintain a progenitor cell status, as cap mesenchyme cells lacking Six2 activity contribute to ectopic nephron tubules, a mechanism dependent on a Wnt9b inductive signal. Taken together, our observations suggest that Six2 activity cell-autonomously regulates a multipotent nephron progenitor population.
Asunto(s)
Proteínas de Homeodominio/genética , Riñón/citología , Nefronas/citología , Organogénesis/genética , Factores de Transcripción/genética , Animales , Diferenciación Celular/genética , Quimera/genética , Quimera/metabolismo , Proteínas de Homeodominio/metabolismo , Inmunohistoquímica , Riñón/embriología , Riñón/fisiología , Células Madre Mesenquimatosas/citología , Células Madre Mesenquimatosas/metabolismo , Ratones , Ratones Endogámicos C57BL , Ratones Transgénicos , Microscopía Confocal , Nefronas/embriología , Nefronas/fisiología , Factores de Transcripción/metabolismo , Activación Transcripcional , Proteínas Wnt/genéticaRESUMEN
Hox transcription factors control morphogenesis along the head-tail axis of bilaterians. Because their direct functional targets are still poorly understood in vertebrates, it remains unclear how the positional information encoded by Hox genes is translated into morphogenetic changes. Here, we conclusively demonstrate that Six2 is a direct downstream target of Hoxa2 in vivo and show that the ectopic expression of Six2, observed in the absence of Hoxa2, contributes to the Hoxa2 mouse mutant phenotype. We propose that Six2 acts to mediate Hoxa2 control over the insulin-like growth factor pathway during branchial arch development.
Asunto(s)
Proteínas de Homeodominio/fisiología , Factores de Transcripción/fisiología , Animales , Secuencia de Bases , Sitios de Unión/genética , Tipificación del Cuerpo , Región Branquial/embriología , Región Branquial/metabolismo , Cartilla de ADN/genética , Femenino , Regulación del Desarrollo de la Expresión Génica , Proteínas de Homeodominio/genética , Proteínas de Homeodominio/metabolismo , Homeostasis , Proteína 5 de Unión a Factor de Crecimiento Similar a la Insulina/genética , Ratones , Ratones Noqueados , Ratones Mutantes , Ratones Transgénicos , Fenotipo , Factor de Transcripción 1 de la Leucemia de Células Pre-B , Embarazo , Regiones Promotoras Genéticas , Transducción de Señal , Somatomedinas/metabolismo , Factores de Transcripción/deficiencia , Factores de Transcripción/genética , Factores de Transcripción/metabolismo , Transcripción GenéticaRESUMEN
During kidney development and in response to inductive signals, the metanephric mesenchyme aggregates, becomes polarized, and generates much of the epithelia of the nephron. As such, the metanephric mesenchyme is a renal progenitor cell population that must be replenished as epithelial derivatives are continuously generated. The molecular mechanisms that maintain the undifferentiated state of the metanephric mesenchymal precursor cells have not yet been identified. In this paper, we report that functional inactivation of the homeobox gene Six2 results in premature and ectopic differentiation of mesenchymal cells into epithelia and depletion of the progenitor cell population within the metanephric mesenchyme. Failure to renew the mesenchymal cells results in severe renal hypoplasia. Gain of Six2 function in cortical metanephric mesenchymal cells was sufficient to prevent their epithelial differentiation in an organ culture assay. We propose that in the developing kidney, Six2 activity is required for maintaining the mesenchymal progenitor population in an undifferentiated state by opposing the inductive signals emanating from the ureteric bud.