TGF/ACTIVIN/NODAL PATHWAY IN INHIBITION OF HUMAN EMBRYONIC STEM CELL DIFFERENTIATION BY MECHANICAL STRAIN

¹ University of Wisconsin - Madison
² University of Wisconsin

^* To whom correspondence should be addressed. E-mail: palecek{at}engr.wisc.edu.

Submitted on August 14, 2007
Revised on September 12, 2007
Accepted on 31 December 2007

	Abstract

Cyclic biaxial mechanical strain has been reported to inhibit human embryonic stem cell differentiation without selecting against survival of differentiated or undifferentiated cells. Here we demonstrate that TGF/Activin/Nodal signaling plays a crucial role in repression of hESC differentiation under mechanical strain. Strain induced transcription of TGF1, Activin A and Nodal, and upregulated Smad2/3 phosphorylation in undifferentiated hESCs. TGF/Activin/Nodal receptor inhibitor SB431542 stimulated differentiation of hESCs cultured under biaxial strain. Exogenous addition of TGF1, Activin A or Nodal alone was insufficient to stimulate hESC self-renewal to replicate behavior of hESCs in presence of strain. However, exogenous TGF1 and Activin A in combination partially replicated the self-renewing phenotype induced by strain but when combined with strain did not further stimulate self-renewal. In presence of mechanical strain, addition of a neutralizing antibody to TGF1 promoted hESC differentiation while inhibition of Activin A by Follistatin promoted hESC differentiation to a lesser extent. Together, these findings reveal that TGF superfamily activation of Smad2/3 is required for repression of spontaneous differentiation under strain and suggest that strain may induce autocrine or paracrine signaling through TGF superfamily ligands.

Key Words: Nodal, TGFbeta, activin A, human embryonic stem cells, mechanotransduction, self-renewal