Hippo/YAP/TAZ Signaling Pathway: The "Mechanosensor" Sensing Stiffness

Introduction: Tissue Stiffness Alters Cell Behavior

The Hippo/YAP/TAZ signaling pathway was originally discovered as a pathway involved in organ size control and suppression of cell proliferation. However, in recent years, it has become clear that the central players of this pathway, YAP (Yes-associated protein) and TAZ (Transcriptional coactivator with PDZ-binding motif), function as Mechanosensors that sense the "stiffness" of the extracellular matrix (ECM) (Nature). In pathologically hardened fibrotic tissue, YAP/TAZ translocate into the nucleus to drive myofibroblast activation, forming a positive feedback loop that promotes further fibrosis.

1. Basic Mechanism of the Hippo Pathway: "On/Off" Control of YAP/TAZ

When Hippo Pathway is "ON": Inactivation of YAP/TAZ

When the Hippo kinase cascade is activated, YAP/TAZ remain in the cytoplasm and do not function.

1. Activation of Upstream Kinases MST1/2: Activated when cell density is high or cell adhesion is established.
2. Phosphorylation of LATS1/2: MST1/2 form a complex with scaffold proteins SAV1 and MOB1, phosphorylating and activating downstream LATS1/2.
3. Phosphorylation of YAP/TAZ: Activated LATS1/2 phosphorylate YAP/TAZ.
4. Sequestration and Degradation in Cytoplasm:
   • Phosphorylated YAP/TAZ bind to 14-3-3 proteins and remain in the cytoplasm.
   • They are further ubiquitinated and degraded by the proteasome.

When Hippo Pathway is "OFF": Activation of YAP/TAZ

When the Hippo kinase cascade is suppressed, YAP/TAZ are not phosphorylated and can translocate into the nucleus.

1. Nuclear Translocation of Unphosphorylated YAP/TAZ: Move into the nucleus instead of staying in the cytoplasm.
2. Binding with TEAD Transcription Factors: Since YAP/TAZ lack DNA binding ability, they bind to transcription factors TEAD1-4.
3. Induction of Target Gene Expression:
   • Proliferation-related genes: CTGF (Connective Tissue Growth Factor), Cyclin D1
   • Fibrosis-related genes: PAI-1, Fibronectin, Collagen

2. Mechanotransduction: YAP/TAZ Sensing Stiffness

The most important characteristic of YAP/TAZ is that they respond to the physical environment outside the cell (especially ECM stiffness) "independently" of the Hippo pathway.

YAP/TAZ Activation on Stiff Substrates

• Soft Substrate (Physiological Stiffness): YAP/TAZ remain in the cytoplasm and are inactive.
• Stiff Substrate (Pathological Stiffness, Fibrotic Tissue): YAP/TAZ translocate into the nucleus and are activated.

Activation Mechanism

1. Adhesion via Integrins: Stiff ECM strengthens cell-ECM adhesion.
2. Tension of Actin Cytoskeleton: RhoA GTPase is activated, forming actin stress fibers.
3. Dephosphorylation of YAP/TAZ: Cytoskeletal tension suppresses LATS1/2, decreasing YAP/TAZ phosphorylation.
4. Nuclear Translocation and Gene Expression: Activated YAP/TAZ bind with TEAD in the nucleus, inducing proliferation and fibrosis genes.

3. Positive Feedback Loop in Fibrosis

The troublesome aspect of YAP/TAZ is the self-amplifying action of "sensing stiffness and making it stiffer."

1. Tissue Injury and Initial Fibrosis: Fibroblasts are activated by inflammation, depositing ECM.
2. ECM Stiffening: Excessive collagen deposition makes the tissue pathologically stiff.
3. Activation of YAP/TAZ: Stiff ECM causes nuclear translocation of YAP/TAZ in fibroblasts.
4. Promotion of Myofibroblast Differentiation: YAP/TAZ induce expression of α-SMA, Collagen, PAI-1, etc.
5. Further ECM Deposition: Myofibroblasts produce massive amounts of collagen, further stiffening the tissue.
6. Persistence of Loop: The vicious cycle of Stiffness → YAP/TAZ Activation → ECM Production → Stiffness continues.

This feedback loop works in coordination with TGF-β signaling to irreversibly progress fibrosis.

4. YAP/TAZ as Therapeutic Targets

Strategies for YAP/TAZ Inhibition

• Verteporfin: A small molecule that directly inhibits YAP-TEAD interaction. Anti-fibrotic effects reported in pulmonary fibrosis models.
• Peptide Inhibitors: Peptides mimicking the TEAD binding domain of YAP/TAZ for competitive inhibition.
• Upstream Targets: Indirect suppression of YAP/TAZ nuclear translocation via RhoA inhibitors or actin polymerization inhibitors.

Challenges

Since YAP/TAZ are also important for stem cell maintenance and tissue regeneration, systemic inhibition carries risks of side effects. Organ-specific delivery and selective inhibition of downstream targets specific to fibrosis are required.

Conclusion

The Hippo/YAP/TAZ signaling pathway is a central mechanism converting the physical signal of "stiffness" into the pathological process of "fibrosis." While TGF-β signaling is the center of "chemical signals," YAP/TAZ is the central effector of "physical signals," and their interaction accelerates fibrosis. Our fibrosis models capture this complex interaction and serve as a platform to evaluate the potential of next-generation therapeutics targeting mechanotransduction.

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References

1. Zhao B, et al. Inactivation of YAP oncoprotein by the Hippo pathway is involved in cell contact inhibition and tissue growth control. Genes Dev. 2007;21(21):2747-2761.
2. 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.
3. Dupont S, et al. Role of YAP/TAZ in mechanotransduction. Nature. 2011;474(7350):179-183.