YAP/TAZ and Tissue Stiffness: Mechanotransduction Targets

Introduction: Why Does Fibrotic Tissue "Self-Exacerbate"?

One of the greatest mysteries in pulmonary fibrosis and liver cirrhosis is "why does fibrosis continue to progress even after the initial inflammation subsides?"

The key to this molecular mechanism lies in the mechanosensors that allow cells to feel the "stiffness" of their surroundings: YAP (Yes-associated protein) and TAZ (Transcriptional co-activator with PDZ-binding motif).

Stiffened tissue activates YAP/TAZ in fibroblasts, further driving collagen production and causing the tissue to become even stiffer. This creates a devastating positive feedback loop: "Stiffness → Fibrosis → More Stiffness". Currently, targeting the physical properties of the tissue itself—a completely different approach from traditional cytokine inhibition (e.g., TGF-β signaling inhibition)—is attracting attention as a novel drug discovery strategy.

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1. Hippo Pathway and the "On/Off" Control of YAP/TAZ

YAP and TAZ are the primary downstream effectors of the Hippo signaling pathway, which controls cell proliferation and organ size.

When the Hippo Pathway is "ON": Inactivation of YAP/TAZ (Normal State)

In healthy tissues where cell density is high and proper cell-cell adhesion is established, the Hippo kinase cascade is activated.

1. Activation of LATS1/2: Upstream kinases (e.g., MST1/2) activate LATS1/2 kinases.
2. Phosphorylation of YAP/TAZ: LATS1/2 phosphorylates YAP and TAZ.
3. Cytoplasmic Sequestration and Degradation: Phosphorylated YAP/TAZ bind to 14-3-3 proteins, are sequestered in the cytoplasm, and are eventually degraded by the proteasome. (As a result, transcription of pro-fibrotic genes is turned OFF)

When the Hippo Pathway is "OFF": Activation of YAP/TAZ (Pathological State)

When the Hippo cascade is suppressed, YAP/TAZ escape phosphorylation.

1. Nuclear Translocation: Unphosphorylated YAP/TAZ translocate into the nucleus.
2. Binding to TEAD Transcription Factors: Since YAP/TAZ lack their own DNA-binding domains, they form complexes with the TEAD family of transcription factors in the nucleus.
3. Induction of Pro-fibrotic Genes: This complex strongly induces the expression of CTGF (Connective Tissue Growth Factor), pro-collagen, α-SMA (alpha-smooth muscle actin), and fibronectin.

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2. Mechanotransduction: The Molecular Machine that Feels Stiffness

The most astonishing property of YAP/TAZ is that, completely independent of the classical Hippo pathway, they directly respond to the physical stiffness of the extracellular matrix (ECM). This is called mechanotransduction.

Activation Mechanism on Stiff Substrates

1. Adhesion and Sensing via Integrins: When cells adhere to ECM stiffened by fibrosis, strong integrin-mediated focal adhesions are formed.
2. Actin Cytoskeletal Tension: RhoA GTPase is activated, forming highly tensioned actin stress fibers within the cell.
3. Nuclear Pore Expansion and YAP/TAZ Translocation: The physical tension of the cytoskeleton is transmitted to the nucleus, deforming nuclear pores and allowing a massive influx of YAP/TAZ.

[IMPORTANT] Fibroblasts cultured on soft physiological gels (elasticity equivalent to normal tissue: ~0.5-1 kPa) keep YAP/TAZ in a dormant state (cytoplasm). However, on hard plastic dishes or fibrotic tissues (tens of kPa or more), YAP/TAZ spontaneously translocate into the nucleus and activate into myofibroblasts, even in the absence of stimulating factors (cytokines).

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3. The Crucial Connection Between YAP/TAZ Research and Ex Vivo Organ Culture (PCLS)

When targeting YAP/TAZ in drug discovery, "the environment in which cells are cultured and evaluated" becomes fatally important.

The Limits of Traditional 2D Culture

When fibroblasts are cultured on standard plastic dishes (stiffness: several Gigapascals), the dish is "abnormally stiff." Consequently, YAP/TAZ is 100% forcibly activated (artifact) simply by culturing them. Under these conditions, true drug efficacy cannot be accurately evaluated.

The Solution: Precision-Cut Tissue Slices (PCLS)

In mechanotransduction drug discovery, the PCLS (Ex vivo organ culture) model is stepping into the spotlight.

• Preservation of Native "Stiffness" and 3D Structure: Because tissue slices taken from animals or patients are cultured as-is, the physical elasticity and 3D organization of the native ECM are relatively well preserved (mechanical damage during slicing and day-by-day shifts in stiffness during culture are unavoidable, so aligning culture duration and assessment timepoints is important).
• Accurate YAP/TAZ Efficacy Evaluation: By allowing compounds to be evaluated in an environment possessing the tissue's true natural stiffness, the "forced YAP activation" artifacts caused by plastic dishes are eliminated, enabling accurate, in vivo-like screening.

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4. Strategies as Drug Targets

Several strategies are being developed to sever the "positive feedback driven by stiffness" of YAP/TAZ.

1. Direct Inhibition of YAP-TEAD Interaction: Small molecules that directly disrupt the YAP-TEAD complex in the nucleus. Beyond tool compounds such as Verteporfin, widely used for preclinical mechanism validation, more recent efforts focus on selective peptidomimetics (including small molecules mimicking the TEAD Ω-loop) that are advancing into clinical development in both oncology and fibrosis.
2. Blocking Upstream Mechanosensors: Approaches that block the "sensing of stiffness" itself using inhibitors of the RhoA/ROCK pathway (which generates cytoskeletal tension) or integrin inhibitors.
3. Suppressing ECM Remodeling: LOXL2 inhibitors and others that physically suppress tissue stiffening.

Challenges: Because YAP/TAZ is also essential for maintaining and regenerating normal tissue stem cells, strong systemic inhibition carries the risk of side effects. Promising approaches include targeted delivery systems to lesion sites or combination therapies (synergetic effects at low doses) with inhibitors of other pathways (e.g., TGF-β/SMAD pathway).

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Conclusion

The Hippo / YAP / TAZ signaling pathway is the central mechanism that translates the physical signal of "stiffness" into the pathological process of "fibrosis." If TGF-β is the main actor of chemical messengers, YAP/TAZ are the main actors of physical messengers. By utilizing biologically relevant 3D models like PCLS, it is expected that the development of next-generation anti-fibrotic drugs targeting this complex mechanotransduction will accelerate.

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Related Articles

• Ex Vivo Precision-Cut Tissue Slices (PCLS) Guide
• Latest Trends in the TGF-β/SMAD Signaling Pathway
• Wnt/β-catenin: Why a Developmental Pathway Drives Fibrosis
• NF-κB Pathway: Strategies to Stop Chronic Inflammation Without Promoting Fibrosis
• Fibrosis Quantification Hub (Including In Vivo / Ex Vivo Assessment)

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References

1. Liu F, et al. "Mechanosignaling through YAP and TAZ drives fibroblast activation and fibrosis." Am J Physiol Lung Cell Mol Physiol. 2015;308(4):L344-357. PubMed
2. Dupont S, et al. "Role of YAP/TAZ in mechanotransduction." Nature. 2011;474(7350):179-183. PubMed
3. Cai X, et al. "Mechanoregulation of YAP and TAZ in Cellular Homeostasis and Disease Progression." Front Cell Dev Biol. 2021;9:673599. PubMed