Transient contractility attenuation reprograms epithelial cells into a protrusion-driven state that drives tissue fluidization

Transient attenuation of actomyosin contractility reprograms confluent epithelial cells into a protrusion-driven, leader-like state by coordinating cytoskeletal, adhesive, and ERK signaling changes, thereby triggering a transition from solid-like to fluid-like tissue behavior.

The Gist

Imagine a city made entirely of living bricks (cells) that are packed tightly together, like a crowded subway car during rush hour. Usually, in this crowded state, everyone is stuck. They can wiggle a little, but they can't really move anywhere because they are jammed against their neighbors. This is what scientists call a "solid-like" tissue.

However, sometimes this city needs to flow like a river to heal a wound or grow into a new shape. This is called "fluidization."

This paper discovers a fascinating trick nature uses to turn a stuck, solid crowd into a flowing, moving river. Here is the story of how they did it and what they found:

The Experiment: The "Relax and Reset" Button

The researchers used a special drug (blebbistatin) to temporarily hit the "pause" button on the muscle-like fibers inside the cells. Think of these fibers as the tiny engines that keep the cells tense and stiff.

1. The Relax: They turned off these engines for 16 hours. The cells went limp and relaxed.
2. The Reset: They washed the drug away and let the cells recover.

The Surprise: You might think that relaxing the cells would make them lazy and slow. Instead, the opposite happened. When the cells woke up, they didn't just go back to normal; they transformed into super-movers.

The Transformation: From "Wallflowers" to "Leaders"

The researchers noticed that these "washed-out" cells (BWO cells) changed in three major ways:

1. They Got Bigger and Stretchier: Like a balloon that has been inflated, these cells became larger and more elongated. They stopped being round and stiff and became long and flexible.
2. They Gripped the Floor Harder: Imagine trying to run on a slippery floor versus a sticky one. These cells suddenly developed super-strong "hands" (adhesions) to grab onto the surface they were walking on. They pulled themselves forward with much more force.
3. They Changed Their GPS: This is the most interesting part. Inside the cells, there is a signaling system (called ERK) that tells them when to move.
   • Normal cells rely on a "neighbor-check" signal (EGFR). They only move if they see a gap or if their neighbors tell them to.
   • The transformed cells switched their GPS to a "leader-mode" (HGFR). Even when they were surrounded by other cells with no gaps, they decided, "I'm going to lead the way!" They started pushing forward aggressively, pulling their neighbors along with them.

The Result: The "Flocking" Effect

When these transformed cells were put back into a crowded crowd, something magical happened.

• Before: The crowd was like a mosh pit where everyone bumps into each other but stays in the same spot (jammed/solid).
• After: The crowd turned into a flock of birds. Because the "leader" cells were pulling forward and stretching out, they created a chain reaction. The whole tissue started to flow in one direction, smoothly and persistently, without getting stuck.

The Computer Simulation: Why It Works

The scientists built a computer model to understand why this happened. They found two ways cells can move:

• The "Push" (Contraction): Imagine a person trying to move by pushing backward against the wall. This tends to make people spin in circles or get stuck in a tight knot.
• The "Pull" (Protrusion): Imagine a person reaching forward with a hand and pulling themselves along. This creates a straight line.

The drug treatment forced the cells to switch from "Pushing" to "Pulling." By reaching forward and pulling, they aligned their bodies with their direction of travel. This alignment is the secret sauce that turns a chaotic crowd into an organized, flowing river.

The Big Picture

This study teaches us that relaxation can be a trigger for action.

In our bodies, tissues often need to soften up (relax) to heal wounds or grow. This paper shows that when cells relax their internal tension, they don't just become floppy; they reprogram themselves to become "leaders." They grab the floor, stretch out, and start pulling the whole group forward.

It's like a traffic jam where, instead of honking and pushing, everyone suddenly decides to take a deep breath, stretch their arms out, and start pulling their cars forward in unison, turning a gridlock into a smooth highway.

Technical Summary

1. Problem Statement

Biological tissues dynamically transition between solid-like (jammed) and fluid-like (unjammed) states, a process known as tissue fluidization. This transition is critical for morphogenesis, wound healing, and collective cell migration. While physical parameters like cell density, shape, and activity are known to influence these states, the active mechano-chemical mechanisms that regulate this transition remain unclear.

Specifically, previous studies showed that transient attenuation of actomyosin contractility (e.g., via Blebbistatin treatment followed by washout) induces a shift from pulsatile, confined motion to coherent, long-range collective flow. However, the underlying cellular reprogramming and signaling pathways driving this "leader-like" state in confluent tissues were not understood. The central hypothesis was that transient mechanical relaxation triggers an intrinsic reprogramming of epithelial cells into a protrusion-driven, leader-like state that fluidizes the tissue.

2. Methodology

The study employed a multidisciplinary approach combining quantitative live-cell imaging, biophysical force measurements, pharmacological perturbations, and computational modeling.

• Experimental System:

  • Cell Model: Madin–Darby Canine Kidney (MDCK) epithelial cells.
  • Treatment Protocol: Cells were treated with Blebbistatin (myosin II inhibitor) for 16 hours, followed by a 4-hour washout/recovery period in drug-free medium. These were termed BWO (Blebbistatin-Washout) cells. Controls were treated with DMSO (DWO).
  • Assays:
    • Morphology & Mechanics: Cell area, elongation, cortical actin staining (Phalloidin), and E-cadherin localization.
    • Adhesion & Force: Trypsin detachment kinetics, centrifugation-based adhesion assays, and Traction Force Microscopy (TFM) on polyacrylamide gels.
    • Signaling: FRET-based biosensors for ERK activity (ERK-KTR), immunostaining for phosphorylated HGFR (pHGFR) and EGFR, and pharmacological inhibition of specific pathways (MEK, HGFR, EGFR, Arp2/3).
    • Collective Dynamics: Monolayer expansion assays, single-cell tracking, Mean Squared Displacement (MSD) analysis, and spatial velocity correlation measurements.

• Computational Modeling:

  • A 2D Cellular Potts Model (CPM) was developed to simulate epithelial monolayers.
  • The model incorporated intercellular adhesion, cell elasticity, and two distinct migration modes: contraction-driven (rear retraction) and protrusion-driven (front extension).
  • Parameters included migration strength ($m_c, m_p$) and cell elasticity ($K_A$).

3. Key Contributions & Results

A. Morphological and Mechanical Reprogramming

• Cell Enlargement and Elongation: BWO cells exhibited a ~146% increase in cell area and 20% increase in elongation in confluent monolayers compared to DWO controls. Interestingly, isolated BWO cells were more rounded, suggesting that elongation is a collective phenomenon driven by cell-cell interactions.
• Cortical Tension vs. Adhesion: BWO cells showed reduced cortical actin (lower tension) but preserved E-cadherin junctions.
• Enhanced Substrate Adhesion: Contrary to the expectation that reduced contractility lowers traction, BWO cells showed 15-fold higher resistance to detachment (centrifugation assay) and a 15% increase in traction stress in monolayers. This was accompanied by increased focal adhesion occupancy (more, smaller FAs) but unchanged turnover rates.

B. Tissue Fluidization and Collective Dynamics

• Transition to Fluid-like Flow: Upon release of confinement, DWO monolayers exhibited pulsatile, oscillatory motion (solid-like). In contrast, BWO monolayers displayed smooth, persistent, long-range collective flow (fluid-like) with a 35% higher leading-edge speed.
• ERK Signaling Rewiring:
  • DWO cells showed oscillatory ERK waves coupled to velocity waves.
  • BWO cells showed a single initial ERK propagation followed by sustained, uniform ERK activation without oscillations.
  • Mechanism: BWO cells switched their ERK regulation from EGFR-dependent (typical of follower cells) to HGFR-dependent (typical of leader cells). Inhibiting HGFR in BWO cells abolished ERK activity and migration, whereas inhibiting EGFR had no effect.
  • Protrusion Coupling: Inhibiting the Arp2/3 complex (actin polymerization) disrupted ERK patterns and migration specifically in BWO cells, confirming that ERK activity in these cells is coupled to actin-based protrusions.

C. Computational Modeling Insights

• The CPM simulations demonstrated that protrusion-driven migration (mimicking BWO) leads to shape-velocity alignment (cells elongate along the direction of motion), whereas contraction-driven migration leads to transverse elongation.
• Protrusion-driven migration promotes convergence of neighbor forces, generating coherent, flocking-like flows.
• The model identified a phase transition from a "caged" (solid) state to a "flocking" (fluid) state as protrusion strength and cell deformability increased.

4. Significance

• Mechanistic Insight: The study provides the first mechanistic link between transient mechanical relaxation and the emergence of a leader-like state in confluent tissues. It challenges the notion that leader cells only exist at tissue boundaries, showing they can be induced intrinsically throughout a monolayer.
• Signaling Plasticity: It reveals that epithelial cells can rewire their mechano-chemical feedback loops, shifting from EGFR-driven oscillatory dynamics to HGFR-driven protrusive dynamics, thereby sustaining migration even in crowded environments.
• Tissue Fluidization: The findings suggest that transient reductions in contractility (e.g., during tissue stretching or injury) act as a general trigger for tissue fluidization, enabling rapid remodeling and repair.
• Theoretical Framework: The integration of experimental data with the CPM offers a predictive framework for understanding how cytoskeletal organization and signaling pathways dictate the macroscopic physical state of biological tissues.

In summary, the paper demonstrates that transient attenuation of contractility reprograms epithelial cells by reducing cortical tension, enhancing substrate adhesion, and rewiring ERK signaling toward an HGFR-dependent, protrusion-driven mode. This reprogramming transforms the tissue from a jammed, solid-like state into a fluid-like, highly motile collective.