Role of numb during Xenopus embryogenesis

ALEJANDRA R. PAGANELLI, SILVIA L. LÓPEZ AND ANDRÉS E. CARRASCO.

Los Cocos, Córdoba. Argentina.

Workshop; Latest Concepts in Developmental Biology; 2006

Role of numb during Xenopus embriogenesis.   Alejandra Paganelli, Silvia López and Andrés Carrasco.   Laboratorio de Embriología Molecular, Instituto de Biología Celular y Neurociencias, Facultad de Medicina, UBA–CONICET. Paraguay 2155, Piso 3 (1121), Buenos Aires, Argentina. e-mail: arpagane@mail.retina.ar     Asymmetric cell division is a conserved mechanism for establishing different cell fates during development. Considerable progress has been made in the understanding of how cells establish polarity during asymmetric cell division and how determinants, in the form of localized proteins and mRNAs are segregated. Genetic studies have been performed in Drosophila neuroblasts, C. elegans zygote and Xenopus blastula stages. In these models, cell fate determinants are localized along an axis of polarity in the dividing cell and are differentially inherited by the daughter cells, which then acquire different fates. In Xenopus, aPKC is localized to the apical membrane and is inherited simply by superficial cells during perpendicularly oriented divisions of frog blastomeres. Solely these superficial cells turn on the bHLH gene ESR6e and prevent them from becoming terminally mature. Postmitotic primary neurons arise only from deep cells. Since that Notch is involved during the binary decision in the notochord/floor plate cell precursors in Xenopus laevis (López et al; 2003, 2005), we want to know if numb is playing a role in this process. The expression pattern of Xenopus numb (X-numb) has not been described until now. First we analyzed the distribution of X-numb transcripts during the development of the frog from gastrulae to tailbud stages by ISH. X-numb messenger is ubiquitously expressed at gastrulae stages (st 10 to st 12). At neurula the messenger is gradually constrained to the neural plate. Tailbud stages show transcripts restricted to the ventricular region of the neural tube where proliferation takes place. To better understand the function of X-numb we blocked its function by injecting an antisense morpholino oligonucleotide against X-numb (X-numb Mo). Transverse sections revealed a reduced notochord with a concomitantly thickened floor plate. This observation resembles our previous results where activation of the Notch pathway down-regulates notochordal markers such as Bra and Chd and decreases the notochord size at the same time. Also, Notch up-regulates two molecules expressed by the floor plate, Plvs (FoxA4a) and Shh, and increases the floor plate size (López et al., 2003). These results are in agreement with the evidences that the cell fate determinant Numb influences developmental cell decisions by antagonizing the Notch signaling, reflecting an asymmetric segregation of numb. These results could be related with the binary decision dependent of Notch during cell fate specification of dorsal midline.