Calreticulin is a secreted BMP antagonist, expressed in Hensen's node during neural induction

Irene De Almeida,Nidia M.M Oliveira,Rebecca A Randall,Caroline S Hill,John M Mccoy,Claudio D Stern

doi:10.1016/j.ydbio.2016.12.001

Irene De Almeida, Nidia M.M Oliveira + Show 4 more

Open Access

https://doi.org/10.1016/j.ydbio.2016.12.001

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Abstract

Hensen's node is the “organizer” of the avian and mammalian early embryo. It has many functions, including neural induction and patterning of the ectoderm and mesoderm. Some of the signals responsible for these activities are known but these do not explain the full complexity of organizer activity. Here we undertake a functional screen to discover new secreted factors expressed by the node at this time of development. Using a Signal Sequence Trap in yeast, we identify several candidates. Here we focus on Calreticulin. We show that in addition to its known functions in intracellular Calcium regulation and protein folding, Calreticulin is secreted, it can bind to BMP4 and act as a BMP antagonist in vivo and in vitro. Calreticulin is not sufficient to account for all organizer functions but may contribute to the complexity of its activity.

Highlights

Neural induction is the process by which signals secreted by the organizer (Hensen's node in amniotes, a structure at the tip of the primitive streak) can instruct cells in the epiblast to change their fate from non-neural to neural plate
A Signal Sequence Trap screen to identify putative secreted factors was performed in yeast as described by Jacobs et al, 1997) (Fig. 1) using a cDNA library constructed by Oligo-dT-primed reverse transcription from mRNA purified from Hensen's nodes of embryos at stage HH3+-4
This latter effect resembles that of BMP antagonists and we show that Calreticulin can bind to BMP4 and inhibit BMP activity in vivo and in vitro

Summary

Introduction

Neural induction is the process by which signals secreted by the organizer (Hensen's node in amniotes, a structure at the tip of the primitive streak) can instruct cells in the epiblast to change their fate from non-neural (eg. epidermis) to neural plate. Known additional factors include FGFs, IGFs and Wnt inhibition, but even a combination of all of these factors is insufficient to mimic the effect of a node graft to the area opaca in the chick (de Almeida et al, 2008; Linker and Stern, 2004), suggesting that other factors are involved. A strain of Saccharomyces cerevisiae with a genomic deletion at the SUC2 locus (Klein et al, 1996a) is unable to secrete invertase and is unable to grow on sucrose or raffinose as the sole carbon source. If the cDNA clone provides the elements required for secretion, the fusion protein is translocated to the secretion pathway, allowing the transformant to grow on sucrose or raffinose as their only source of carbon (Jacobs et al, 1997)

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