Human Mesenchymal Stem Cells Migration on Matrices with Distinct Elasticity Gradient Magnitudes

Abstract

Adult mesenchymal stem cells (MSCs) respond to extracellular niche elasticity, which varies dramatically between tissues that MSCs inhabit. Similarly, as MSCs egress from bone marrow and hone to tissues, they may encounter stiffness gradients brought on either by pathological conditions, e.g. myocardial infarction ∼8.7±1.5kPa/mm, or through normal tissue variation, e.g. muscle ∼0.6±0.9kPa/mm. We have recently shown that MSCs can undergo directed migration in response to shallow, physiological (∼1kPa/mm) stiffness gradients before differentiating, suggesting the importance of spatial changes in stiffness. Such gradients, however, contain aphysical ranges, e.g. 1-15kPa, and more refined gradients of both range and gradient strength that mimic tissue interfaces are needed to better understand how mechanical cues dictate MSC migration versus differentiation. Using a polydimethylsiloxane microchannel mixer, we generated a polymer solution with a one-dimensional crosslinker concentration of constant monomer and photoinitiator but varying crosslinker. Photopolymerizing the solution inside the device yields a 3mm wide hydrogel with varying mechanical properties. Stiffness gradients of varying strength (1-30kPa/mm) and range (0.1-100 kPa) are achieved by varying the relative concentration of crosslinker from the input solutions. MSCs responded to stiffness gradients with a physiological range, e.g. mimicking the myotendenous junction, but of varying strength, i.e. 1-30kPa/mm. However, migration velocities of MSCs on gels of varying gradient strength were similar. Cell morphology was stiffness dependent with cells exhibiting increased spread areas on the stiffer regions, suggesting that the previously observed correlation between substrate mechanics, cell motility, and morphology exists over a physiological stiffness range but is independent of gradient strength. Efforts to define optimal myogenic stiffness and studies with C2C12 muscle cells are ongoing. These findings imply that MSCs in vivo may contribute better to repairs in stiffer regions of tissues where they may preferentially accumulate.