Objective To investigate the mechanical responses of mitochondrial morphology to extracellular matrix stiffness in human mesenchymal stem cells (hMSCs) and the role of AMP-activated protein kinase (AMPK) in the regulation of mitochondrial mechanoresponses.

Methods Two polyacrylamide (PAAm) hydrogels, a soft one with a Young's modulus of 1 kPa and a stiff one of 20 kPa, were prepared by changing the monomer concentrations of acrylamide and bis-acrylamide. Then, hMSCs were cultured on the soft and stiff PAAm hydrogels and changes in mitochondrial morphology were observed using a laser confocal microscope. Western blot was performed to determine the expression and activation of AMPK, a protein associated with mitochondrial homeostasis. Furthermore, the activation of AMPK was regulated on the soft and stiff matrixes by AMPK activator A-769662 and the inhibitor Compound C, respectively, to observe the morphological changes of mitochondria.

Results The morphology of the mitochondria in hMSCs showed heterogeneity when there was a change in gel stiffness. On the 1 kPa soft matrix, 74% mitochondria exhibited a dense, elongated filamentous network structure, while on the 20 kPa stiff matrix, up to 63.3% mitochondria were fragmented or punctate and were sparsely distributed. Western blot results revealed that the phosphorylated AMPK (p-AMPK)/AMPK ratio on the stiff matrix was 1.6 times as high as that on the soft one. Immunofluorescence assay results revealed that the expression of p-AMPK was elevated on the hard matrix and showed nuclear localization, which indicated that the activation of intracellular AMPK increased continuously along with the increase in extracellular matrix stiffness. When the hMSCs on the soft matrix were treated with A-769662, an AMPK activator, the mitochondria transitioned from a filamentous network morphology to a fragmented morphology, with the ratio of filamentous network decreasing from 74% to 9.5%. Additionally, AMPK inhibition with Compound C promoted mitochondrial fusion on the stiff matrix and significantly reduced the generation of punctate mitochondria.

Conclusion Extracellular matrix stiffness regulates mitochondrial morphology in hMSCs through the activation of AMPK. Stiff matrix promotes the AMPK activation, resulting in mitochondrial fission and the subsequent fragmentation of mitochondria. The impact of matrix stiffness on mitochondrial morphology can be reversed by altering the level of AMPK phosphorylation.