YAP/TAZ activity in stromal cells prevents ageing by controlling cGAS-STING.

Ageing is intimately connected to the induction of cell senescence1,2, but why this is so remains poorly understood. A key challenge is the identification of pathways that normally suppress senescence, are lost during ageing and are functionally relevant to oppose ageing3. Here we connected the structural and functional decline of ageing tissues to attenuated function of the master effectors of cellular mechanosignalling YAP and TAZ. YAP/TAZ activity declines during physiological ageing in stromal cells, and mimicking such decline through genetic inactivation of YAP/TAZ in these cells leads to accelerated ageing. Conversely, sustaining YAP function rejuvenates old cells and opposes the emergence of ageing-related traits associated with either physiological ageing or accelerated ageing triggered by a mechano-defective extracellular matrix. Ageing traits induced by inactivation of YAP/TAZ are preceded by induction of tissue senescence. This occurs because YAP/TAZ mechanotransduction suppresses cGAS-STING signalling, to the extent that inhibition of STING prevents tissue senescence and premature ageing-related tissue degeneration after YAP/TAZ inactivation. Mechanistically, YAP/TAZ-mediated control of cGAS-STING signalling relies on the unexpected role of YAP/TAZ in preserving nuclear envelope integrity, at least in part through direct transcriptional regulation of lamin B1 and ACTR2, the latter of which is involved in building the peri-nuclear actin cap. The findings demonstrate that declining YAP/TAZ mechanotransduction drives ageing by unleashing cGAS-STING signalling, a pillar of innate immunity. Thus, sustaining YAP/TAZ mechanosignalling or inhibiting STING may represent promising approaches for limiting senescence-associated inflammation and improving healthy ageing.

Broadly Applicable Hydrogel Fabrication Procedures Guided by YAP/TAZ-Activity Reveal Stiffness, Adhesiveness, and Nuclear Projected Area as Checkpoints for Mechanosensing.

Mechanical signals are pivotal ingredients in how cells perceive and respond to their microenvironments, and to synthetic biomaterials that mimic them. In spite of increasing interest in mechanobiology, probing the effects of physical cues on cell behavior remains challenging for a cell biology laboratory without experience in fabrication of biocompatible materials. Hydrogels are ideal biomaterials recapitulating the physical cues that natural extracellular matrices (ECM) deliver to cells. Here, protocols are streamlined for the synthesis and functionalization of cell adhesive polyacrylamide-based (PAA-OH) and fully-defined polyethyleneglycol-based (PEG-RGD) hydrogels tuned at various rigidities for mechanobiology experiments, from 0.3 to >10 kPa.  The mechanosignaling properties of these hydrogels are investigated in distinct cell types by monitoring the activation state of YAP/TAZ. By independently modulating substrate stiffness and adhesiveness, it is found that although ECM stiffness represents an overarching mechanical signal, the density of adhesive sites does impact on cellular mechanosignaling at least at intermediate rigidity values, corresponding to normal and pathological states of living tissues. Using these tools, it is found that YAP/TAZ nuclear accumulation occurs when the projected area of the nucleus surpasses a critical threshold of approximatively 150 µm2 . This work suggests the existence of distinct checkpoints for cellular mechanosensing.

Simple yet effective methods to probe hydrogel stiffness for mechanobiology.

In spite of tremendous advances made in the comprehension of mechanotransduction, implementation of mechanobiology assays remains challenging for the broad community of cell biologists. Hydrogel substrates with tunable stiffness are essential tool in mechanobiology, allowing to investigate the effects of mechanical signals on cell behavior. A bottleneck that slows down the popularization of hydrogel formulations for mechanobiology is the assessment of their stiffness, typically requiring expensive and sophisticated methodologies in the domain of material science. Here we overcome such barriers offering the reader protocols to set-up and interpret two straightforward, low cost and high-throughput tools to measure hydrogel stiffness: static macroindentation and micropipette aspiration. We advanced on how to build up these tools and on the underlying theoretical modeling. Specifically, we validated our tools by comparing them with leading techniques used for measuring hydrogel stiffness (atomic force microscopy, uniaxial compression and rheometric analysis) with consistent results on PAA hydrogels or their modification. In so doing, we also took advantage of YAP/TAZ nuclear localization as biologically validated and sensitive readers of mechanosensing, all in all presenting a suite of biologically and theoretically proven protocols to be implemented in most biological laboratories to approach mechanobiology.