Matrix stiffening promotes perinuclear clustering of mitochondria.

Mechanical cues from the tissue microenvironment, such as the stiffness of the extracellular matrix, modulate cellular forms and functions. As numerous studies have shown, this modulation depends on the stiffness-dependent remodeling of cytoskeletal elements. In contrast, very little is known about how the intracellular organelles such as mitochondria respond to matrix stiffness and whether their form, function, and localization change accordingly. Here, we performed an extensive quantitative characterization of mitochondrial morphology, subcellular localization, dynamics, and membrane tension on soft and stiff matrices. This characterization revealed that while matrix stiffness affected all these aspects, matrix stiffening most distinctively led to an increased perinuclear clustering of mitochondria. Subsequently, we could identify the matrix stiffness-sensitive perinuclear localization of filamin as the key factor dictating this perinuclear clustering. The perinuclear and peripheral mitochondrial populations differed in their motility on soft matrix but surprisingly they did not show any difference on stiff matrix. Finally, perinuclear mitochondrial clustering appeared to be crucial for the nuclear localization of RUNX2 and hence for priming human mesenchymal stem cells towards osteogenesis on a stiff matrix. Taken together, we elucidate a dependence of mitochondrial localization on matrix stiffness, which possibly enables a cell to adapt to its microenvironment.