Understanding the extracellular forces that determine cell fate and maintenance

Abstract

Stem cells interpret signals from their microenvironment while simultaneously modifying the niche through secreting factors and exerting mechanical forces. Many soluble stem cell cues have been determined over the past century, but in the past decade, our molecular understanding of mechanobiology has advanced to explain how passive and active forces induce similar signaling cascades that drive self-renewal, migration, differentiation or a combination of these outcomes. Improvements in stem cell culture methods, materials and biophysical tools that assess function have improved our understanding of these cascades. Here, we summarize these advances and offer perspective on ongoing challenges.

Keywords: Biomechanics; Extracellular matrix; Mechanobiology; Stem cells; Stiffness.

Fig. 1.

The stem cell as a mathematical integrator. A stem cell can integrate several input types to result in an output that is the (often amplified) summation of all cues it receives. Representative inputs and outputs are shown for a generic stem cell during development. Such cues can be chemical (e.g. soluble or cell-surface signaling molecules) or physical – involving the generation or modification of intra- or intercellular forces.

Fig. 2.

Externally applied and biomaterial-induced forces. Forces can be applied to cells using multiple experimental techniques (see also Table 1). (A) Magnetic twisting cytometry can locally apply forces to cells via twisting a bead on the surface of the cell. (B) Substrate deformations can be used to modulate cell response by applying forces dynamically through cyclic stretching, and (C) by applying fluidic shear to cell surface. Separately, (D) biomaterials can also be used to apply forces to cells by varying substrate stiffness. (E) The adhesive area to which a cell will attach and spreading can affect the ability of a cell to contract against that surface. When multiple cells are patterned together, their intracellular forces must balance the forces of the adjoining cell, which are transmitted across the cell-cell junction. (F) Biomaterials can also have temporal and spatial gradients. For example, the thickness of the biomaterial can be changed as a function of location. There can also be temporal changes such as dynamic stiffening where the substrate modulus is changed during the culture process. Finally, biomaterials can be fabricated with gradients in crosslinking density changing their presentation as a function of location. (G) Whereas the other methods use continuous surfaces, micropillars of varying height effectively change surface rigidity, i.e. the longer the post, the softer apparent rigidity, to affect a change in cell behavior. Surfaces can still be actively modulated through the addition of magnetic wires within the posts.

Fig. 3.

Integrating and converting biophysical signals. Forces can drive stem cell responses using one or more of multiple mechanisms used to convert these forces into biochemical signals. These include: (A) actin-myosin contraction regulation; (B) focal adhesion (FA)-based signaling mechanisms; (C) force-sensitive transcription factor localization; (D) stretch-activated channels (SAC) causing ion flux changes; and (E) nuclear-associated protein signaling resulting from force transduction into the nucleus via SUN and emerin causing chromatin unfolding. In all cases, these mechanisms result in transcriptional changes, translational changes (not indicated) or protein activity changes in the cytoplasm (not indicated).