Elsevier

Vitamins & Hormones

Volume 87, 2011, Pages 341-365
Vitamins & Hormones

Chapter Sixteen - Transforming Growth Factor-Beta Superfamily in Mouse Embryonic Stem Cell Self-Renewal

https://doi.org/10.1016/B978-0-12-386015-6.00035-4Get rights and content

Abstract

Embryonic stem (ES) cells are pluripotent cells that maintain the capability of undifferentiated self-renewal in culture. As mouse ES cells have the capacity to give rise to all the tissues of the body, they are an excellent developmental biology model system and a model for regenerative therapies. The extracellular cues and the intracellular signaling cascades that regulate ES cell self-renewal and cell-fate choices are complex and actively studied. Many developmental signaling pathways regulate the ES cell phenotype, and their intracellular programs interact to modulate the gene networks controlling ES cell pluripotency. This review focuses on the current understanding and outstanding questions of the roles of the transforming growth factor-beta-related signaling pathways in regulating pluripotency and differentiation of mouse ES cells. The complex dichotomic roles of bone morphogenetic protein signaling in maintaining the undifferentiated state and also inducing specific cell fates will be reviewed. The emerging roles of Nodal signaling in ES cell self-renewal will also be discussed.

Introduction

Embryonic stem (ES) cells are derived from the inner cell mass of the blastocyst (Evans and Kaufman, 1981, Martin, 1981). As the inner cell mass gives rise to the epiblast and subsequently to all three germ layers of the embryo, ES cells maintain this property of pluripotency and can differentiate into most cell fates. The capability of ES cells to have sustained pluripotent growth during long-term in vitro culture produces a valuable system for both studying early embryonic development and investigating key signaling pathways controlling pluripotency and differentiation. The capacity for an ES cell to remain undifferentiated and germline competent is a function of the intrinsic transcriptional machinery that directs ES cell self-renewal. Transcriptional regulatory networks are being uncovered in ES cells, including essential DNA-binding proteins such as Oct4, Sox2, and Nanog (Orkin et al., 2008). The self-renewal, pluripotency, and differentiative capacity of ES cells are regulated by numerous extracellular stimuli of the culture conditions as well as the autocrine signaling produced by the ES cells themselves. The downstream molecular mechanisms by which different signaling pathways affect stem cell pluripotency and interact with other intracellular pathways are an active and growing area of developmental biology and stem cell research.

The transforming growth factor-beta (TGF-beta)-related signaling pathways have complex roles in regulating the pluripotency and cell fate of ES cells. The TGF-beta pathway is composed to two main branches, bone morphogenetic proteins (BMP) and Activin/Nodal/TGF-beta subfamilies. While BMP signaling is a well-studied pluripotency pathway in mouse ES cells, the role of Activin/Nodal/TGF-beta signaling is less clear and appears to affect pluripotency indirectly. This chapter will focus on the current understanding of the roles of TGF-beta-related signaling in mouse ES cell biology.

Section snippets

Overview of TGF-Beta-Related Signaling

Work from many models systems has constructed a canonical model for the molecular cascades that direct TGF-beta-related signaling from the extracellular ligand to the nucleus to regulate changes in gene expression. This literature is extensive and cannot be comprehensively detailed here; this chapter will provide a basic overview of the signaling pathways and components most relevant to mouse ES cell renewal and differentiation.

ES Cell Regulation by the BMP Pathway

BMPs were originally identified for their ability to induce bone formation (Urist, 1965); however, a large body of work has shown that these secreted factors play an active role in the prenatal and postnatal development of nearly all tissues and organ systems, including the nervous system, somites, lung, kidney, skin, muscle, gonads, and the hematopoietic system. Moreover, genetic and developmental studies have shown that BMP signaling plays a vital role in body plan development and pattern

Roles in pluripotency

Recent work has been undertaken to define the roles of Smad2 signaling in ES cells. ES cells have an active Smad2 signaling axis which can be manipulated in vitro through recombinant ligands and pharmacological inhibitors (Watabe and Miyazono, 2009). Importantly, this signaling activity is present even under serum-free conditions (Galvin et al., 2010), strongly suggesting that Smad2 activation in undifferentiated ES cells is via an autocrine signaling mechanism. Of the Activin/Nodal/TGF-beta

Interactions of TGF-Beta Signaling with Other Pluripotency Pathways

While cross talk occurs between the BMP and Nodal pathways in ES cells, these pathways also interact with other signaling pathways. As discussed previously, BMP signaling may support ES cell self-renewal via inhibition of both ERK and p38 MAPK pathways (Qi et al., 2004). Additionally, BMP signaling has been shown to interact with the Wnt pathway. In both human and mouse ES cells, activation of the canonical Wnt pathway is sufficient to maintain pluripotency (Sato et al., 2004). Stimulation of

Human ES cells

Mouse and human ES cells have distinct self-renewal mechanisms. Unlike mouse ES cells, which can utilize the combination of LIF and BMP signaling to maintain stemness, human ES cells utilize alternative signaling pathways to retain their pluripotency (Daheron et al., 2004). Activin signaling through Smad2 phosphorylation maintains human ES cells in their undifferentiated state and sustains the expression of pluripotency genes (Beattie et al., 2005, James et al., 2005). Recombinant Activin or

Conclusions and Future Directions

The TGF-beta superfamily elicits responses from a wide-ranging variety of cells, from kidney cells to neurons to cancer cells to stem cells. Understanding the intrinsic factors that create a unique pluripotent environment in ES cells is essential to discerning how BMP and Nodal signals influence that pluripotent state. While BMP induces expression of the Id factors and acts to inhibit differentiating-inductive signals (Ying et al., 2003, Ying et al., 2008), its effect on ES cell pluripotency is

Acknowledgment

We wish to acknowledge Stanton Fernald for his work on the illustrations.

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