Dynamic compression regulates cell-matrix adhesions and intracellular signaling proteins of human mesenchymal stem cells in 3D collagen environment

Abstract

Background and objectives Mechanical signal is critical to stem cell function and tissue construct maturation in functional tissue engineering. Cell-matrix adhesions are one of the major mechanosensors cells use to convert mechanical signals into intracellular biochemical signaling cascades which lead to particular functional responses. In this study, our objective is to investigate the effects of dynamic compression on cell-matrix adhesions and associated intracellular signaling proteins of hMSCs in 3D collagen environment.

Methods We used microencapsulation technique to embed cells in 3D collagen environment, forming disc-shaped hMSC-collagen constructs. We used a custom-made manipulator-based loading system to impose dynamic compression on the hMSC-collagen constructs at 1 Hz and 10% peak-to-peak cyclic deformation. By immunofluorescence, we labeled integrins αv, β1, α5, FAKpY397, vinculin and fibronectin in loaded and unloaded constructs. Stained tissue sections were then viewed under confocal microscope.

Results We imposed 10 min compression to observe the effect of loading to integrin αv/β1 and mechanosensitive proteins FAKpY397 and vinculin. One minute after compression, integrin αv formed larger clusters while integrin β1 appeared to have no observable increased clustering. FAKpY397 formed distinct puncta which colocalized with enlarged αv integrin clusters but not with integrin β1 in these loaded constructs. Cell also formed intense vinculin puncta which strongly colocalized with integrin αv upon loading, indicating vinculin recruitment to enlarged integrin αv clusters. This observation indicates stress development at these integrin αv adhesion sites. Five minute after compression, FAKpY397 concentrated as large intense spot near the nucleus, indicating that FAKpY397 converged to trigger downstream signaling pathways. These results suggest that cells sense the compression signals by activation of FAK signaling pathway through enhanced clustering of integrin αv but not integrin β1. Integrin α5 was previously reported to mature into elongated adhesions in cell-derived matrices. We imposed 9h compression and 7-day daily 5h compression to observe any integrin α5 adhesion maturation. We observed that hMSCs were presenting more integrin α5 at cell surface after 9-hour dynamic compression. We also found colocalization of integrin α5 and fibronectin in these loaded constructs. Interestingly, cells formed elongated mature α5 integrin-containing adhesions after daily 5-hour compression for 7 days.

Conclusions Our results showed that cells sense compression signals through FAK signaling pathway mediated integrin αv but not by integrin β1 in 3D collagen environment. We also found that external force may induce adhesion maturation in a 3D environment.