Plastin-3 membrane recruitment drives cell-in-cell invasion during entosis

This study identifies that ROCK1 activation drives cell-in-cell invasion during entosis by recruiting the actin-bundling protein Plastin-3 to the plasma membrane, thereby facilitating the cortical tension and cytoskeletal remodeling necessary for one cell to engulf another.

The Gist

The Big Picture: When Cells Eat Their Neighbors

Imagine a crowded city where the buildings (cells) are usually peaceful neighbors. Sometimes, however, one building decides to physically push its way inside another building, living there like a squatter. In biology, this strange phenomenon is called Entosis (or "Cell-in-Cell" invasion).

Usually, this happens when cells are stressed or starving. But for a long time, scientists didn't know exactly how a cell could force its way into another living cell, or what specific "tools" it used to do it. This paper solves that mystery by finding the "on switch" and the "construction crew" responsible for this invasion.

---

The "On Switch": ROCK1

Think of a cell's skeleton (the cytoskeleton) as a network of ropes and pulleys that gives the cell its shape and strength. To invade a neighbor, a cell needs to tighten these ropes to become super-strong and rigid, allowing it to push its way in.

The scientists discovered that a specific protein called ROCK1 acts as the master switch for this process.

• The Experiment: They turned on ROCK1 artificially (like flipping a light switch) in breast cancer cells.
• The Result: Immediately, the cells started invading their neighbors. They didn't need to be starving or stressed; just flipping the ROCK1 switch was enough to start the invasion.
• The Proof: When they turned the switch off (using a drug called Y27632), the invasion stopped completely.

Analogy: Imagine a construction crew (the cell) that needs a crane to lift a heavy beam. ROCK1 is the person who turns the crane's engine on. Without the engine, the beam stays put. With the engine on, the beam moves.

The "Construction Crew": Plastin-3 (PLS3)

Once the engine (ROCK1) is running, the cell needs a specific tool to actually grab and hold onto the actin ropes to make them strong enough to push. The scientists found this tool: a protein called Plastin-3 (PLS3).

• What it does: PLS3 is like a super-glue or a bundle-tie. It grabs loose strands of the cell's skeleton (actin) and bundles them tightly together. This creates a rigid, reinforced structure right at the cell's edge.
• The Discovery: When ROCK1 was turned on, PLS3 rushed to the cell's outer wall (the membrane) to do its job.
• The Test:
  • Adding more PLS3: The cells invaded more often.
  • Removing PLS3: Even with the "engine" (ROCK1) running, the cells couldn't invade. They were like a car with a running engine but no wheels.

Analogy: If ROCK1 is the engine, PLS3 is the tires. You can have a powerful engine, but if you don't have tires to grip the road, the car (the cell) can't move forward to invade the neighbor.

The Drug: Narciclasine

The researchers also tested a drug called Narciclasine. Think of this drug as a remote control that accidentally hits the "ROCK1" button.

• When they gave this drug to cancer cells, it triggered the whole invasion process.
• They watched the cells in real-time and saw them round up, push into neighbors, and sometimes even swallow multiple neighbors at once (creating "Russian nesting doll" structures of cells inside cells).
• Crucially, they found that this drug didn't just kill the cells; it specifically triggered this mechanical invasion process.

Why Does This Matter? (The "Why Should I Care?" Part)

1. It's not just about stress:
For years, scientists thought cells only did this when they were dying or starving. This paper shows that cells can be programmed to do this just by turning on a specific mechanical switch. It's a deliberate behavior, not just a desperate accident.

2. The "Bad Guy" Connection:
The scientists looked at real breast cancer patients. They found that patients with high levels of PLS3 (the "tire glue") had a higher risk of their cancer coming back (relapse) and lower survival rates.

• The Takeaway: If a tumor has a lot of PLS3, it might be better at invading other cells, surviving, and spreading. This makes PLS3 a potential target for new drugs.

3. A New Way to Study Cancer:
Because the researchers found a way to trigger this invasion on command (using the ROCK1 switch or the Narciclasine drug), they now have a "playground" to study exactly how cancer cells fight each other, survive, or die inside tumors.

Summary in One Sentence

This paper reveals that cancer cells can be forced to invade their neighbors by flipping a mechanical switch (ROCK1), which recruits a specific "glue" protein (PLS3) to bundle the cell's skeleton, turning the cell into a rigid invader that can push its way inside other cells.

Technical Summary

1. Problem Statement

Entosis is a cell-in-cell (CIC) invasion process where one living cell actively invades a neighboring host cell. This phenomenon plays a critical role in tumor evolution, cell competition, and therapeutic resistance. Despite nearly two decades of research, the molecular mechanisms governing the initiation of entosis remain poorly understood.

• The Gap: While the RhoA–ROCK1 signaling axis and actomyosin contractility are known to be required for entosis, it is unclear if ROCK1 activation alone is sufficient to trigger the process or if it merely permits it under stress conditions.
• The Challenge: Entosis typically occurs at low frequencies and is often triggered by indirect stressors (e.g., glucose deprivation, matrix detachment), making it difficult to dissect the specific signaling events that drive the physical internalization of a cell without confounding stress responses.
• The Goal: To identify a direct, pharmacologically targetable trigger for entosis and to uncover the downstream cytoskeletal effectors that physically enable membrane remodeling and cell internalization.

2. Methodology

The authors employed a multi-modal approach combining cell biology, pharmacology, proteomics, and advanced imaging:

• Cell Models: Primarily used MCF7 breast cancer cells, along with ZR75-1, EFM19, MCF10A (non-transformed), and HeLa cells.
• ROCK1 Activation Strategies:
  • Genetic: Transfection with constitutively active GFP-ROCK1-Δ3 (lacking the autoinhibitory domain).
  • Pharmacological: Treatment with Narciclasine, a known activator of RhoA/ROCK1 signaling.
• Inhibition/Knockdown: Used ROCK1 inhibitor (Y27632), Myosin II inhibitor (Blebbistatin), Pan-caspase inhibitor (zVAD-fmk), and siRNA-mediated silencing of ROCK1 and Plastin-3 (PLS3).
• Imaging Techniques:
  • Live-cell time-lapse microscopy: To observe dynamic invasion, cell fates (death, escape, division), and motility.
  • Confocal Microscopy: Immunofluorescence for markers like LAMP1, β-catenin, E-cadherin, pMLC2, and PLS3.
  • Correlative Light and Electron Microscopy (CLEM) & Volumetric EM (vEM): To visualize ultrastructural details of entotic vacuoles and nested CIC structures.
• Proteomics: Mass spectrometry (DIA-MS) to profile global proteomic changes in narciclasine-treated cells, followed by Gene Ontology (GO) and Reactome pathway enrichment analysis.
• Clinical Correlation: Survival analysis using public breast cancer transcriptomic datasets to correlate PLS3 expression with relapse-free survival.

3. Key Contributions & Results

A. ROCK1 Activation is Sufficient to Trigger Entosis

• Constitutive Activation: Expression of the constitutively active ROCK1 mutant (GFP-ROCK1-Δ3) was sufficient to induce entosis in MCF7 cells, recapitulating canonical morphological features (crescent-shaped host nucleus, internalized cell).
• Pharmacological Induction: Treatment with Narciclasine rapidly induced entosis in a dose-dependent manner (peaking at 1 µM).
• Mechanism: Narciclasine-induced entosis was strictly dependent on ROCK1 activity. Pre-treatment with the ROCK1 inhibitor Y27632 or genetic silencing of ROCK1 completely abolished entotic structure formation.
• Contractility: The process required actomyosin contractility, evidenced by the rapid phosphorylation of Myosin Light Chain 2 (pMLC2) and the abolition of entosis by Blebbistatin (myosin II inhibitor).
• Independence from Apoptosis: Pre-treatment with the caspase inhibitor zVAD-fmk did not block entosis, ruling out caspase-mediated ROCK1 cleavage or apoptotic engulfment as the primary driver.

B. Identification of Plastin-3 (PLS3) as a Critical Effector

• Proteomic Screening: Mass spectrometry revealed that narciclasine treatment upregulated cytoskeletal and adhesion proteins, specifically identifying Plastin-3 (PLS3/T-plastin) and Annexin-A6.
• Differential Expression: PLS3 was highly expressed in breast cancer cell lines (MCF7, ZR75-1, EFM19) but low/absent in non-transformed MCF10A cells. Consistently, narciclasine failed to induce entosis in MCF10A cells.
• Membrane Recruitment: Upon ROCK1 activation, PLS3 redistributed from a diffuse cytoplasmic pattern to the plasma membrane, specifically at sites of cell-cell contact and invasion.
• Functional Validation:
  • Overexpression: Ectopic expression of PLS3-GFP increased the frequency of entotic events.
  • Silencing: Depletion of PLS3 via siRNA significantly reduced (by ~75%) narciclasine-induced entosis, confirming PLS3 is essential for the process.

C. Ultrastructural and Clinical Insights

• Complex Architectures: Volumetric EM revealed complex "nested" CIC structures (e.g., a cell inside a cell inside a cell) and multi-cell invasions, suggesting strong ROCK1 activation drives repeated internalization events.
• Cell Fates: Time-lapse imaging showed that internalized cells could undergo entotic cell death (ECD), escape, divide, or remain quiescent.
• Clinical Relevance: High PLS3 expression in breast cancer patients was significantly associated with reduced relapse-free survival, suggesting PLS3-driven entosis may contribute to tumor aggressiveness and poor prognosis.

4. Significance

• Mechanistic Breakthrough: This study establishes ROCK1 activation as a direct, sufficient trigger for entosis, moving beyond the view that entosis is merely a secondary response to stress.
• Novel Molecular Link: It identifies Plastin-3 (PLS3) as the critical downstream effector that couples ROCK1-mediated contractility to the physical actin-bundling required for membrane remodeling and cell internalization.
• Therapeutic Implications: The findings suggest that the ROCK1-PLS3 axis is a druggable pathway in breast cancer. Targeting this axis could potentially modulate cell competition and tumor evolution.
• Prognostic Value: The correlation between high PLS3 levels and poor patient survival highlights PLS3 as a potential biomarker for aggressive breast cancer phenotypes.

Conclusion

Prapa et al. demonstrate that pharmacological or genetic activation of ROCK1 is sufficient to drive entosis through the rapid recruitment of Plastin-3 to the plasma membrane. PLS3 acts as a calcium-sensitive actin-bundling protein that provides the necessary cortical tension and cytoskeletal reorganization for one cell to invade another. This work provides a tractable platform for studying entosis and identifies a specific mechanochemical pathway (ROCK1-PLS3) that may be exploited in cancer therapy.