Professor Sean Palecek
Regulating human embryonic stem cell self-renewal and differentiation by mechanical and chemical cues
Embryonic stem cells (ESCs) hold tremendous promise in tissue engineering and regenerative medicine applications because of their unique combination of two properties, pluripotency and an extremely high proliferative capacity. Theoretically, almost unlimited supplies of cells and tissues could be generated from a single clonal source if we can regulate ESC growth and differentiation. Hurdles facing utilization of ESCs in regenerative medicine include a lack of effective systems that permit robust, large scale culture and expansion of undifferentiated cells and a lack of reliable methods to differentiate ESCs to desired developmental lineages. Several critical factors regulate whether a human ESC (hESC) chooses to self-renew or differentiate. Soluble signals bind receptors and stimulate chemical pathways that lead to global changes in gene transcription and cell differentiation state. Likewise, immobilized extracellular matrix cues synergize with soluble signals to control cell signaling and differentiation. Cell-cell communication is also an important consideration in hESC culture; at low cell densities cell growth rates diminish while at high cell densities spontaneous differentiation occurs. Finally, mechanical signals have recently been shown to affect self-renewal and differentiation. I will discuss examples that illustrate how each of these microenvironmental stimuli can be incorporated in culture systems to expand or differentiate hESCs along desired lineages. Applying cyclic biaxial mechanical strain, along with appropriate chemical self-renewal factors, to undifferentiated hESCs stimulates their self-renewal. Mechanical strain induces expression of TGF superfamily ligands, which appear to act through a paracrine or autocrine signaling mechanism to maintain hESCs in a self-renewing state at high densities. We have developed materials that confine hESC colonies to specific sizes and shapes to assess how colony morphology affects self-renewal and directed differentiation toward therapeutically-relevant lineages, including cardiac myocytes. Finally, the temporal presentation of soluble differentiation factors (e.g. retinoic acid and bone morphogenic proteins) can be utilized to obtain pure populations of stratified epithelial progenitors, which can then differentiate to terminal cells such as epidermal keratinocytes.