Mesenchymal stem cell durotaxis depends on substrate stiffness gradient strength

TLDR

Mesenchymal stem cells sense and respond to tissue elasticity, and shallow stiffness gradients within tissues can direct their migration before differentiation. The study aims to generate finely controlled stiffness gradients to elucidate how MSC migration is mechanically regulated in health and disease. Polyacrylamide substrates were fabricated with three gradient strengths—physiological (1 Pa/µm), pathological (10 Pa/µm), and step (≥100 Pa/µm)—spanning 1–12 kPa elastic moduli. MSCs preferentially migrated toward stiffer regions, with velocity increasing with gradient strength; actin integrity was essential for migration while microtubules were needed for directionality, and contractile agonists or cytoskeletal drugs diminished directed movement, suggesting MSCs may accumulate in high‑modulus tissue sites to aid repair.

Abstract

Mesenchymal stem cells (MSCs) respond to the elasticity of their environment, which varies between and within tissues. Stiffness gradients within tissues can result from pathological conditions, but also occur through normal variation, such as in muscle. MSC migration can be directed by shallow stiffness gradients before differentiating. Gradients with fine control over substrate compliance - both in range and rate of change (strength) - are needed to better understand mechanical regulation of MSC migration in normal and diseased states. We describe polyacrylamide stiffness gradient fabrication using three distinct systems, generating stiffness gradients of physiological (1 Pa/μm), pathological (10 Pa/μm), and step change (≥ 100Pa/μm) strength. All gradients spanned a range of physiologically relevant elastic moduli for soft tissues (1-12 kPa). MSCs migrated to the stiffest region on each gradient. Time-lapse microscopy revealed that migration velocity correlated directly with gradient strength. Directed migration was reduced in the presence of the contractile agonist lysophosphatidic acid (LPA) and cytoskeleton-perturbing drugs nocodazole and cytochalasin. LPA- and nocodazole-treated cells remained spread and protrusive on the substrate, while cytochalasin-treated cells did not. Nocodazole-treated cells spread in a similar manner to untreated cells, but exhibited greatly diminished traction forces. These data suggest that a functional actin cytoskeleton is required for migration whereas microtubules are required for directed migration. The data also imply that, in vivo, MSCs may preferentially accumulate in regions of high elastic modulus and make a greater contribution to tissue repairs in these locations.