Activin A, a member of the TGF-β superfamily, has been shown to mimicking nodal. Activins interact with two types of cell surface transmembrane receptors, which have intrinsic serine/threonine kinase activities in their cytoplasmic domains. Activin A binds to the type II receptor and initiates a cascade reaction that leads to the recruitment, phosphorylation, and activation of type I activin receptors. This activation is followed by phosphorylation of SMADs2/3, two of the cytoplasmic SMAD proteins. SMAD3 then translocates to the nucleus and interacts with SMAD4 through multimerization, resulting in the activation of transcription factor complexes responsible for specific gene expression.
Using the embryoid body differentiation model of mouse embryonic stem cells, first studies showed that endoderm cells could be induced by treatment with activin A. It has to be mentioned that activin A also induced the formation of neuronal extensions and neurofilament proteins in PC12 cells pointing towards the involvement of activin A in neural differentiation. The potential of activin A to induce embryonic stem cells into both pancreatic and neural lineages has been demonstrated.
It became evident that the induction of human embryonic stem cells into definitive endoder (DE) cells required a sophisticated schedule including the sequential application of activin A and other endodermand pancreas-specific growth factors. D'Amour et al. showed that human embryonic stem-derived DE cells (Sox17+/Cxcr4+) could be differentiated into foregut endoderm (Hnf4a+/Hnf1b+), pancreatic endoderm (Nkx6.1+/Pdx1+) and endocrine precursors (Ngn3+/Nkx2.2+) and finally, hormone expressing endocrine cells. Because in the 2006 study many of the embryonic stem cell terminal derivatives showed expression of more than one hormone, i.e., insulin and glucagon coexpression, the lack of glucose-dependent insulin release in such cells pointed to an immature stage of in vitro differentiated cells. After advancement of the differentiation protocol and transplantation of human embryonic stem-derived pancreatic precursor cells into diabetic mice, in vivo differentiation and maturation facilitated the development of glucose-sensing beta-like cells that normalized high blood glucose levels. These data suggest that the in vivo maturation of appropriate activin A- and growth factor-induced pancreatic progenitors (rather then transplantation of fully differentiated cells) is advantageous to generate functional human embryonic stem-derived beta-like cells. However, it also means that entire in vitro methods to direct embryonic stem cells into terminal stages of islet-like clusters are not yet available.
The directed differentiation of embryonic stem cells into DE progenitors includes selective induction and enrichment of DE cells by activin A. However, numerous studies provided evidence that successful differentiation of embryonic stem cells into DE and pancreatic progenitors are significantly affected by parameters, such as contaminating constituents in culture media during activin A application, activin A concentration, time and duration of treatment, and differentiation models used.
Whereas activin A in murine embryonic stem cells clearly induced differentiation into mesendoderm, endoderm and specifically DE cells, in human embryonic stem cells activin A at low concentration is also able to maintain the undifferentiated pluripotent state. Specifically, concentrations of activin A between 5 and 20 ng/ml in serum-free media have been used to keep human embryonic stem cells undifferentiated, and also long-term culture of human embryonic stem cells could be achieved by 50 ng/ml activin A. Obviously, the application of activin A to embryonic stem cells critically affects the decision between proliferation (pluripotency) and differentiation (i.e., endoderm induction) in a concentrationand stage-dependent manner. For example, sustained nodal expression in mouse embryonic ste cells maintained Oct4 levels and prevented the differentiation of DE cells, and activin treatment of embryonic stem cells for a long period resulted in up-regulation of both, Oct4 and Sox17 expression in a dose-dependent manner.
• Sulzbacher S, et al. Activin A-induced differentiation of embryonic stem cells into endoderm and pancreatic progenitors—the influence of differentiation factors and culture conditions[J]. Stem Cell Reviews and Reports, 2009, 5(2): 159-173.