Extracellular fluid viscosity regulates human mesenchymal stem cell lineage and function.

Area of Science:

• Biomaterials Science
• Cell Biology
• Mechanobiology

Background:

• Human mesenchymal stem cells (hMSCs) are known to respond to substrate mechanical properties like stiffness and viscoelasticity.
• The impact of extracellular fluid viscosity on hMSC behavior, particularly in conjunction with substrate properties, remains largely unexplored.

Purpose of the Study:

• To investigate the role of extracellular fluid viscosity as a physical cue influencing hMSC function.
• To determine how fluid viscosity affects hMSC differentiation and phenotype on substrates with varying stiffness and viscoelasticity.

Main Methods:

• Utilized varying substrate stiffness and viscoelasticity with controlled extracellular fluid viscosity.
• Analyzed hMSC differentiation pathways, including actin remodeling, membrane tension, ion channel activity, and key protein translocations.
• Assessed macrophage phenotype polarization in response to hMSC behavior under different viscosity conditions.

Main Results:

• Elevated fluid viscosity biased hMSCs toward an osteogenic phenotype, independent of substrate properties.
• Increased viscosity induced Arp2/3-dependent actin remodeling, enhanced NHE1 activity, and promoted hMSC spreading.
• Fluid viscosity modulated membrane tension, activating TRPV4 channels, calcium influx, and the RhoA/ROCK/YAP pathway, leading to RUNX2 nuclear translocation.
• hMSCs cultured in elevated viscosity on soft gels favored an M2 macrophage phenotype.

Conclusions:

• Extracellular fluid viscosity is a critical physical cue that dictates hMSC differentiation, imparting an osteogenic memory.
• Elevated viscosity promotes osteogenesis and induces an immunosuppressive M2 macrophage phenotype, highlighting its potential in regenerative medicine and tissue engineering.