Mesenchymal stem cell durotaxis depends on substrate stiffness gradient strength

Abstract

Mesenchymal stem cells (MSCs) respond to niche elasticity, which varies between and within tissues. Stiffness gradients result from pathological conditions but also occur through normal variation, e.g. muscle. MSCs undergo directed migration even in response to shallow stiffness gradients before differentiating. More refined gradients of both stiffness range and strength are needed to better understand mechanical regulation of migration in normal and disease pathologies. We describe polyacrylamide stiffness gradient fabrication using three distinct systems that generate stiffness gradients of physiological (1 Pa/µm), pathological (10 Pa/µm), and step (≥ 100Pa/um) strength spanning physiologically relevant stiffness for most soft tissue, i.e. 1–12 kPa. MSCs migrated to the stiffest region for each gradient. Time-lapse microscopy revealed that migration velocity scaled directly with gradient strength. Directed migration was reduced in the presence of the contractile agonist lysophosphatidic acid (LPA) and cytoskeletal-perturbing drugs nocodazole and cytochalasin; LPA- and nocodazole-treated cells remained spread and protrusive, while cytochalasin-treated cells did not. Untreated and nocodazole-treated cells spread in a similar manner, but nocodazole-treated cells had greatly diminished traction forces. These data suggest that actin is required for migration whereas microtubules are required for directed migration. The data also imply that in vivo, MSCs may have a more significant contribution to repairs in stiffer regions where they may preferentially accumulate.

Stem cell migration and mechanotransduction on linear stiffness gradient hydrogels

“…Previous methods of reproducing in vivo stiffness gradients in culture systems are either complex (19,30,34,36,37) or lack the stiffness or gradient range (9,30,32) to interrogate the physiological mechanical landscape fully (Table S1). Therefore, we developed a method to fabricate planar PA hydrogels with linear stiffness gradients capable of spanning both biologically relevant ranges and disease conditions.…”

“…When mimicking the stiffness of neural (∼1 kPa), muscle (∼12 kPa), and bone (∼30 kPa) tissues (15,16), substrate stiffness can induce differentiation toward those specific tissue types. Furthermore, stem cells will migrate toward regions of higher stiffness in a process known as "durotaxis" (17)(18)(19), whereas neurons have shown preference for softer regions (20,21). Additionally, some cancer cell lines are inversely sensitive to substrate stiffness and thus show markedly different migration phenotypes (22)(23)(24).…”

“…Indeed, studies performed with gradient photomasks for UV photopolymerization have shown poor reproducibility and gradient linearity (19). Recent work has demonstrated the ability to build stiffness gradients in PA based on diffusion-driven gradients, although these methods require photolithography and microfluidics (34).…”

Measurement of dynamic cell-induced 3D displacement fields in vitro for traction force optical coherence microscopy

“…It has been shown that mechanical cues, such as substrate stiffness or local stresses, have bearing on cellular behavior in many contexts [1][2][3][4][5][6]. For example, it has been shown that extracellular matrix (ECM) stiffness can promote the onset and progression of cancer [7][8][9][10], and that metastatic cancer cells exert greater traction forces than their healthy counterparts [11].…”

Bioengineering approaches to guide stem cell-based organogenesis

“…Stiffness gradients have also been achieved using specially designed photomasks to control spatially the light exposure experienced by UV-crosslinked materials. This approach has been used to reconstitute natural stiffness variations present in normal or infarcted myocardial tissue, and to investigate mesenchymal stem cell durotaxis in culture (Tse and Engler, 2011;Vincent et al, 2013).…”