Human cell F-actin density differentially influences trogocytosis and phagocytosis by Entamoeba histolytica

This study demonstrates that human cell F-actin density differentially regulates Entamoeba histolytica ingestion, where reduced cortical F-actin specifically enhances phagocytosis while trogocytosis is suppressed across all cytoskeletal mutants, revealing a more complex relationship than previously suggested by artificial stiffening models.

The Gist

The Big Picture: A Parasite with Two Eating Styles

Imagine a microscopic parasite called Entamoeba histolytica (let's call it "The Nibbler"). This parasite causes a nasty disease called amoebiasis, which leads to severe diarrhea and can even damage the liver.

To survive and cause damage, The Nibbler eats human cells. But it has two very different ways of eating:

1. Trogocytosis (The "Nibbler"): Instead of swallowing a whole cell, The Nibbler takes tiny bites out of the cell's surface, like a mouse taking nibbles out of a piece of cheese. Eventually, the cell dies from all the bites.
2. Phagocytosis (The "Gobbler"): The Nibbler swallows the entire human cell whole, like a snake eating a mouse.

For a long time, scientists thought the Nibbler chose its eating style based on how stiff or soft the human cell was. The theory was: "If the cell is hard and stiff, I'll swallow it whole. If it's soft and squishy, I'll just take a few bites."

The Experiment: Changing the Human Cell's "Skeleton"

The researchers in this paper wanted to test this theory, but they didn't want to use fake cells or glue to make cells stiff (which is like putting a cast on a finger to see how it feels). Instead, they wanted to see what happens when they change the internal skeleton of the human cell naturally.

Think of a human cell like a bouncy castle. The "bounciness" and shape are held up by a network of ropes and beams inside called F-actin (part of the cell's cytoskeleton).

The scientists used a genetic tool (CRISPR) to cut the "ropes" (genes) that control this skeleton in human T-cells. They made several different versions of these cells:

• Some had loose, floppy skeletons (less F-actin).
• Some had super-tight, rigid skeletons (more F-actin).
• Some had weirdly shaped skeletons.

Then, they let The Nibbler try to eat these different versions.

The Surprising Results

The results were a bit more complicated than the simple "stiff vs. soft" theory suggested.

1. The "Nibble" (Trogocytosis) Got Worse for Everyone
No matter what the scientists did to the human cell's skeleton—whether they made it loose, tight, or weird—the Nibbler became worse at taking bites.

• The Analogy: Imagine you are trying to take a bite out of a cookie. If the cookie is fresh, it's easy. If you put the cookie in the freezer, in the microwave, or wrap it in foil, it becomes harder to bite into. In this study, any change to the cookie's structure made it harder for the Nibbler to nibble. It didn't matter if the cookie got harder or softer; the Nibbler just couldn't get a good grip.

2. The "Gobble" (Phagocytosis) Was a Different Story
When it came to swallowing whole cells, the stiffness did matter, but in a specific way:

• The "Tight" Skeleton (MYPT1 mutant): When the human cell had a very dense, tight skeleton (like a bouncy castle with the air pumps turned up to max), The Nibbler swallowed more of them.
• The "Loose" Skeleton (ROCK1 mutant): When the human cell had a sparse, loose skeleton (like a deflated, floppy bouncy castle), The Nibbler swallowed fewer of them.
• The Analogy: It's like trying to swallow a beach ball. If the ball is fully inflated and firm (tight skeleton), it's easier to grab and gulp down. If the ball is half-deflated and floppy (loose skeleton), it's slippery and hard to get a good grip on to swallow whole.

Why Does This Matter?

This study teaches us two big lessons:

1. Living things are complex: Previous studies used "fake" stiffening (like gluing cells together) and concluded that "stiff = swallow, soft = nibble." This study shows that in real, living cells, the relationship is much more complicated. The cell's internal skeleton is dynamic and alive, not just a static block of rubber.
2. The "Nibble" is sensitive: The fact that any change to the cell's skeleton made the Nibbler worse at nibbling suggests that the "nibbling" process is very sensitive to the cell's natural state. It's not just about how hard the cell is; it's about how the cell's internal structure is organized.

The Takeaway

The researchers found that while the "Gobbler" (phagocytosis) prefers firm, well-structured targets, the "Nibbler" (trogocytosis) struggles whenever the target cell's internal structure is messed with, regardless of whether it's too hard or too soft.

This helps scientists understand how this parasite causes disease and suggests that to stop it, we might need to look at how human cells maintain their internal "skeletons" to resist being nibbled or swallowed. It's a reminder that biology is rarely as simple as "hard vs. soft"; it's about the complex, living dance of the cell's internal machinery.

Technical Summary

1. Problem Statement

Entamoeba histolytica is a parasitic amoeba responsible for amoebiasis, a significant cause of diarrheal disease and liver abscesses. The parasite kills host cells through two distinct ingestion mechanisms:

1. Trogocytosis: "Cell-nibbling," where the amoeba pinches off and internalizes fragments of the host cell membrane and cytoplasm, eventually killing the host cell.
2. Phagocytosis: The engulfment of entire host cells.

Previous studies, often utilizing artificial targets (e.g., beads) or artificially stiffened cells (e.g., glutaraldehyde-fixed red blood cells), suggested a simple mechanical model: stiffer targets are phagocytosed, while softer targets undergo trogocytosis. However, these models fail to account for the dynamic nature of the living cytoskeleton. The authors sought to determine if the native, dynamic organization of the host cell's actin cytoskeleton (specifically F-actin density) differentially regulates these two ingestion pathways in a more physiologically relevant context.

2. Methodology

The study employed a CRISPR interference (CRISPRi) approach to generate human Jurkat T cell mutants with specific perturbations in the Rho-pathway, which regulates actin cytoskeletal dynamics.

• Cell Line Engineering:
  • Jurkat T cells were stably transduced with a dCas9-KRAB system to enable transcriptional repression.
  • Seven canonical Rho-pathway genes were individually knocked down (KD): CFL1, MYPT1, Rac1, Rac2, RhoA, ROCK1, and ROCK2.
  • An ANPEP (Alanyl Aminopeptidase) non-targeting sgRNA served as the negative control.
  • Clonal line 2D10 was selected for consistent dCas9-KRAB expression.
• Validation of Knockdowns:
  • RT-qPCR and Western Blotting confirmed significant reduction in mRNA and protein levels for the target genes.
  • Phalloidin Staining & Super-resolution Microscopy (Airyscan): Used to visualize and characterize F-actin organization and cortical density in the mutants compared to controls.
• Ingestion Assays:
  • Trogocytosis Assay: Co-incubation of E. histolytica trophozoites (labeled with CellTracker Green) with Jurkat cells (labeled with DiD). Quantified via Imaging Flow Cytometry to measure the percentage of amoebae performing ingestion and the intensity of internalized host material (Low vs. High trogocytosis).
  • Phagocytosis Assay: Co-incubation of amoebae (CellTracker Green) with Jurkat cells (Hoechst-labeled nuclei). Quantified via Live Microscopy to count phagocytic events, phagocytic cups, and the number of ingested cells per amoeba.

3. Key Results

A. Cytoskeletal Perturbations

Knockdown of Rho-pathway genes resulted in distinct and abnormal F-actin morphologies:

• MYPT1 KD: Exhibited dense, ring-like cortical F-actin structures with minimal protrusions (indicating increased actomyosin contractility/rigidity).
• ROCK1 KD: Showed thin cortical actin networks with elongated filopodia (indicating reduced contractility).
• Rac1/RhoA KD: Displayed reduced F-actin stability and sparse networks.
• CFL1 KD: Showed dense peripheral actin with large ruffles (reduced turnover).

B. Impact on Trogocytosis

• Universal Reduction: Amoebae performed quantitatively reduced levels of trogocytosis on all Rho-pathway knockdown mutants compared to control cells.
• Independence from Stiffness: This reduction occurred regardless of whether the mutant had increased (MYPT1) or decreased (ROCK1) cortical F-actin density.
• Shift in Intensity: While the percentage of amoebae attempting ingestion remained similar, the efficiency dropped; amoebae performed "low-level" trogocytosis more frequently and "high-level" trogocytosis less frequently on mutants.

C. Impact on Phagocytosis

• Inverse Correlation with F-actin Density: Phagocytosis efficiency showed a direct relationship with cortical F-actin density, but in an inverse manner to the "stiffness" hypothesis.
  • MYPT1 KD (High F-actin density): Amoebae performed increased levels of phagocytosis.
  • ROCK1 KD (Low F-actin density): Amoebae performed decreased levels of phagocytosis.
  • Other Mutants: Rac2, RhoA, and others showed no significant change in phagocytosis efficiency compared to controls.
• Phagocytic Cup Dynamics: ROCK1 mutants (low actin) exhibited the highest number of stalled phagocytic cups, suggesting that insufficient cortical actin density prevents the completion of the phagocytic cup closure.

4. Key Contributions

1. Decoupling Trogocytosis and Phagocytosis: The study demonstrates that these two ingestion pathways are regulated by distinct mechanical cues from the host cell. Trogocytosis is highly sensitive to any disruption of the cytoskeletal network, whereas phagocytosis is specifically sensitive to cortical F-actin density.
2. Refutation of Simple Stiffness Models: The findings contradict the simple "stiff = phagocytosis, soft = trogocytosis" model derived from artificial targets. Instead, the dynamic remodeling of the living cytoskeleton is critical.
3. Native Cell Context: This is the first study to systematically vary host cytoskeletal organization using genetic knockdowns in native cells, rather than relying on chemical cross-linking or artificial beads, providing a more physiologically accurate model of host-pathogen interaction.
4. Mechanistic Insight: The data suggests that while a robust cytoskeleton is required for efficient trogocytosis (likely for the "nibbling" mechanics), a dense cortical actin network specifically facilitates the engulfment required for phagocytosis, possibly by providing the necessary resistance for the amoeba to "grip" and close the phagocytic cup.

5. Significance

• Pathogenesis: Understanding how host cell mechanics influence E. histolytica ingestion is crucial for understanding tissue invasion and abscess formation in amoebiasis.
• Broader Biological Context: The work sheds light on the emerging field of eukaryotic trogocytosis, suggesting it is a conserved but complex process that relies on the dynamic mechanoresponse of the target cell, rather than static physical properties.
• Therapeutic Implications: By identifying that cytoskeletal integrity is a key factor in preventing or facilitating amoebic ingestion, future therapeutic strategies could potentially target host cytoskeletal regulators to modulate the parasite's ability to invade and kill host cells.

In conclusion, the paper establishes that human cell F-actin organization differentially influences amoebic trogocytosis and phagocytosis, highlighting that the dynamic nature of the living cytoskeleton creates a more complex regulatory landscape than previously understood through artificial models.