Enhancing differentiation of human induced pluripotent stem cells (2014)

Human induced pluripotent stem cells (iPS cells) resemble embryonic stem cells (ES cells) and can differentiate into all tissue types of our body. Hence human iPS cells serve as suitable candidates for disease modeling, drug development and cellular therapy. However, the differentiation efficiency of human iPS cells towards specific lineages is rather poor - particularly towards mesodermal and endodermal lineages, such as hematopoietic cells, hepatocytes and pancreatic cells. In this study, we aim to search for new approaches, which could enhance the differentiation efficiency of human iPS cells. Cell fusion exists physiologically in our body (e.g. muscle and liver cells), to make cells acquire increasing cellular function. In vitro, upon fusion of somatic cells with pluripotent stem cells, the somatic genome is reprogrammed into a pluripotent state. Hence, cell fusion is often used as a tool for studying reprogramming. We hypothesize that the somatic genome in such hybrids might contribute to their differentiation potential and thus enhances pluripotent stem cell differentiation. Therefore, we fused human iPS cells with hematopoietic stem cells (HSC) from cord blood to generate iPS/somatic cell hybrids, referred to as iPS hybrids. Shortly after fusion, iPS hybrids acquired similar characteristics as parental iPS cells, including gene expression profiles and epigenetic signatures. Different from parental iPS cells, iPS hybrids showed a prominent differentiation bias towards hematopoietic lineages but also towards other mesendodermal lineages. Interestingly, we found that the strong mesendodermal differentiation bias of iPS hybrids was already initiated at the primitive streak stage, when the first decision of mesendoderm or ectoderm specification is made. The prominent primitive streak development in iPS hybrids was directed by NODAL signaling. Accordingly, inhibition of NODAL signaling blunted the mesendodermal differentiation bias. Biomaterials can mimic properties of in vivo extracellular matrix (ECM) components and provide an in vitro cellular “niche”. Therefore biomaterials are expected to direct differentiation of pluripotent stem cells towards specific lineages. Similarly, small molecules activate or inhibit specific signaling pathways, which are crucial for cell fate specification and can affect differentiation capacity of pluripotent stem cells. To this end, we investigated whether the biomaterial Poly(L-lactide-co-D,L-lactide) (LR704) and the compound 1-ethyl-2-benzimidazolinone (EBIO), an activator of Ca2+-activated potassium channels (SKCas), affect human iPS cell differentiation. We found that both of LR704 and EBIO showed rather unspecific effects on the differentiation of human iPS cells. Thus, in the context studied here cell fusion of iPS cells with somatic cells is superior to biomaterials and small molecules in enhancing iPS cell differentiation. Taken together, we propose that cell fusion of iPS cells with somatic cells provides an efficient strategy for enhancing iPS cell differentiation towards cell types, which are notoriously difficult to obtain.