A stiff hydrogel enhanced early bone formation in mice through the cellular deformation sensor PIEZO1

A stiff hydrogel enhanced early bone formation in mice through the cellular deformation sensor PIEZO1

On September 6, Advanced Science published a study examining how material placed in a bone defect changes cell behavior. In wild-type mice with a 1-millimeter femoral defect and hindlimb unloading, 15-percent GelMA hydrogel produced more new trabecular bone and collagen by day seven than 5-percent GelMA. This was an early observation point, and the key comparisons included five mice per group.

Cells in damaged bone contact the matrix, a network of molecules surrounding them. The authors tested whether the stiffness of this network could direct cells toward bone or fat differentiation. Primary mouse mesenchymal stem cells were cultured on gels with different levels of resistance to deformation: 2 and 25 kPa. The stiffer material increased markers of bone differentiation, while the softer material led to greater lipid droplet accumulation under conditions that induced fat differentiation.

PIEZO1 is a channel in the cell membrane that allows calcium to enter in response to deformation. GsMTx4, a blocker of mechanosensitive channels, weakened the effect of the stiff material. Bone differentiation on the stiff gel was also reduced in cells from mice in which Piezo1 had been deleted in mesenchymal progenitor cells.

The authors traced the pathway from calcium to the Stc2 gene. They examined CaMKII, a protein that relays calcium signals: inhibiting it reduced the levels of active SP1 and STC2. SP1 binds to a DNA region upstream of Stc2 and initiates its transcription. The authors modified this DNA region, measured its activity in cells, and separately tested SP1 binding; the results supported this part of the pathway.

STC2 is a protein secreted by cells. Adding STC2 shifted cells toward bone differentiation even when PIEZO1 was pharmacologically inhibited. In a separate experiment, soft 5-percent GelMA containing STC2 enhanced early signs of bone formation in the defect.

The genetic control experiments in animals were conducted in a separate series without hindlimb unloading. In mice with Piezo1 deleted in mesenchymal progenitor cells, neither the soft nor the stiff gel enhanced bone formation; the difference between 5-percent and 15-percent GelMA disappeared.