RAB-35 regulates distinct steps of trogocytosis in the biting and bitten cell

This study identifies the small GTPase RAB-35 as a dual-function regulator of trogocytosis in *C. elegans*, where it promotes lobe digestion in biting endodermal cells by removing PIP2 and facilitates membrane scission in bitten primordial germ cells via the ESCRT complex.

The Gist

Imagine a biological process called trogocytosis. The name comes from Greek and literally means "cell nibbling." It's a bit like a Pac-Man game, but instead of eating dots, one cell takes a bite out of another living cell, chews it up, and digests it.

This paper investigates a very specific, pre-planned "nibbling" event that happens inside the tiny worm C. elegans. Here, special "endodermal" cells (the Biting Cells) nibble off little pieces of "primordial germ cells" (the Bitten Cells, or PGCs). These pieces are called lobes.

The scientists wanted to know: How does the cell know when to bite, how to cut the piece off, and how to digest it?

They discovered a tiny molecular manager called RAB-35 that acts like a foreman, but it does two completely different jobs depending on which side of the fence it's standing on.

The Two Jobs of the Foreman (RAB-35)

Think of the biting cell and the bitten cell as two neighbors building a fence between them. The goal is to cut a piece of the neighbor's wall (the lobe) and bring it inside your own house to recycle.

Job 1: Inside the Biting Cell (The Endoderm)

The Role: The "Cleanup Crew Chief."
Once the Biting Cell has successfully cut off the piece of the neighbor's wall, it needs to break it down.

• What RAB-35 does: It rushes to the scene of the "bite" and acts like a demolition expert. It removes a specific chemical "glue" called PIP2 from the membrane surrounding the new piece.
• The Analogy: Imagine the new piece is wrapped in sticky tape (PIP2) that keeps it stuck to the outside world. RAB-35 comes in with a solvent to dissolve that tape. Once the tape is gone, the cell's internal recycling plant (lysosomes) can move in and digest the piece.
• The Result: Without RAB-35 in the biting cell, the "tape" stays on, the recycling plant can't work, and the nibbled piece just sits there, undigested and floating around.

Job 2: Inside the Bitten Cell (The PGC)

The Role: The "Self-Detachment Specialist."
This is the surprising part. The scientists found that the cell getting bitten also needs RAB-35 to help the process.

• What RAB-35 does: It works with a team of molecular scissors called the ESCRT complex. Together, they help sever the connection between the lobe and the main body of the cell.
• The Analogy: Imagine the bitten cell is a balloon with a long string attached to it. Usually, you'd think the person holding the scissors (the biting cell) does all the cutting. But this paper shows that the balloon (the bitten cell) actually has its own internal mechanism (RAB-35 + ESCRT) that helps pull the string tight and snap it from the inside.
• The Result: Without RAB-35 in the bitten cell, the "string" doesn't snap cleanly. The lobe gets stuck, and the biting cell can't fully take it away.

The "Glue" Problem (PIP2)

The paper also explains why this glue (PIP2) is so important.

• In the Biting Cell, RAB-35 removes the glue so digestion can happen.
• In the Bitten Cell, if RAB-35 is missing, the glue stays too strong. This makes it hard for the internal scissors (ESCRT) to cut the membrane. It's like trying to cut a rubber band that has been super-glued; it just won't snap cleanly.

The Big Picture

Before this study, scientists thought the "Biting Cell" did all the work: it recognized the target, cut it off, and ate it.

This paper reveals a team effort.

1. The Bitten Cell isn't just a passive victim; it actively helps cut itself off using RAB-35 and the ESCRT team.
2. The Biting Cell uses RAB-35 to clean up the "glue" so it can digest the prize.

Why Does This Matter?

This isn't just about worms. This "nibbling" happens in humans too:

• Immune System: Immune cells nibble parts of bacteria or viruses to learn how to fight them.
• Cancer: Sometimes tumor cells nibble parts of healthy cells to hide from the immune system.
• Development: Our bodies use this to clean up old cells as we grow.

By understanding how RAB-35 coordinates this "nibbling" from both sides, scientists might find new ways to stop cancer cells from hiding or help the immune system fight infections more effectively.

In short: RAB-35 is the universal manager that ensures the "nibbling" process goes smoothly, acting as a cleanup crew for the eater and a self-detachment specialist for the eaten.

Technical Summary

1. Problem Statement

Trogocytosis is a cellular process where one cell ("biting") nibbles off and internalizes a fragment of another cell ("bitten"). While distinct from phagocytosis (which consumes whole cells or debris), the molecular mechanisms governing the specific steps of trogocytosis—particularly the scission (severing) of the fragment from the target cell and its subsequent digestion—are poorly understood.

A specific developmental model exists in C. elegans where endodermal cells bite off "lobes" (extensions rich in organelles) from Primordial Germ Cells (PGCs). Previous work identified a scission complex in the biting cell (involving Dynamin and LST-4/SNX9), but it remained unclear:

1. Whether the bitten cell (PGC) actively participates in the scission process.
2. The specific molecular regulators required for the digestion of trogocytosed material.
3. How the biting and bitten cells coordinate these events.

2. Methodology

The authors employed a combination of genetic, molecular, and imaging techniques using C. elegans:

• Genetic Screen: A chemical mutagenesis screen was performed to identify mutants with "persistent lobes" (PGC lobes that fail to be bitten off or digested) in L1 larvae.
• Whole Genome Sequencing (WGS): Used to identify causal mutations in the isolated mutants.
• CRISPR/Cas9 Genome Editing: Used to create specific alleles, including:
  • rab-35(xn224): A functional null (nonsense mutation).
  • rab-35(xn225): An endogenously tagged allele with a ZF1 degron and HA epitope for conditional protein depletion.
  • Split-GFP systems (GFP11 tagged to rab-35 or tsg-101) combined with cell-specific GFP1-10 expression to visualize protein localization in specific cell types (Endoderm vs. PGCs).
• Conditional Depletion (ZF1/ZIF-1 System): Used to deplete RAB-35 specifically in endodermal cells (biting) or PGCs (bitten) to determine cell-autonomous functions.
• Fluorescence Recovery After Photobleaching (FRAP): Used to distinguish between lobes that are still connected to the PGC (scission defect) versus those that are detached but undigested (digestion defect).
• Lipid Imaging: Used PIP2-binding reporters (PH domain of PLC$\delta$1) to monitor phosphatidylinositol 4,5-bisphosphate (PIP2) levels on lobe membranes.
• RNAi: Used to partially knock down ESCRT complex components (tsg-101, vps-4) to test for genetic interactions with RAB-35.

3. Key Contributions & Results

A. Identification of RAB-35 as a Central Regulator

• A genetic screen identified RAB-35 (a small GTPase) as a critical regulator of PGC lobe trogocytosis.
• rab-35 null mutants exhibit a highly penetrant "persistent lobe" phenotype (92% of L1 larvae), where lobes fail to be cleared.
• The phenotype is temperature-sensitive, suggesting partial redundancy with other pathways.

B. Distinct Roles in Biting vs. Bitten Cells

Using cell-specific depletion, the authors demonstrated that RAB-35 has separable, non-redundant functions in the two cell types:

1. In the Biting Cell (Endoderm): RAB-35 is required primarily for the digestion of internalized lobes. Depletion here leads to lobes that are successfully scissed but accumulate as undigested debris.
2. In the Bitten Cell (PGC): RAB-35 is required for scission. Depletion here leads to lobes that remain physically attached to the PGC cell body (Type I lobes), indicating a failure to sever the membrane.

C. Mechanism of Digestion: PIP2 Turnover

• In the biting cell, RAB-35 enrichment occurs on the membrane surrounding the cut lobe after scission.
• RAB-35 functions with its downstream effectors CNT-1 (an ArfGAP) and ARF-6 to promote the turnover of PIP2.
• In rab-35 mutants, PIP2 levels remain abnormally high on the endodermal membrane surrounding the lobe, blocking the maturation of the phagosome/trogosome and preventing digestion. This mirrors the mechanism of phagosome maturation in cell corpse clearance.

D. Mechanism of Scission: The Bitten Cell's Active Role

• The study provides evidence that the bitten cell (PGC) actively participates in its own scission.
• In PGCs lacking RAB-35, scission fails. The authors hypothesize that RAB-35 promotes scission by regulating membrane topology or recruiting the ESCRT complex.
• ESCRT Interaction: Partial knockdown of ESCRT components (tsg-101, vps-4) significantly enhances the scission defects of rab-35 mutants in PGCs. This suggests RAB-35 and ESCRT function in a coordinated pathway (potentially parallel or sequential) to sever the membrane from the inside (reverse topology).
• Unlike the biting cell's Dynamin-dependent scission, the PGC's contribution likely involves ESCRT-mediated membrane fission.

E. Coordination of Scission

• The data implies a "two-sided" scission mechanism: The biting cell assembles a Dynamin/LST-4 complex to cut from the outside, while the bitten cell utilizes RAB-35 and ESCRT to facilitate cutting from the inside. Both processes must be coordinated for successful trogocytosis.

4. Significance

• Novel Mechanism of Trogocytosis: This paper defines a molecular pathway for trogocytosis that is distinct from, yet related to, phagocytosis. It highlights that trogocytosis is a cooperative process requiring active participation from the target cell.
• Dual Function of RAB-35: It reveals that RAB-35 is not just a recycling or maturation factor but a bifunctional regulator that controls both the physical severing of a membrane (in the target) and the subsequent digestion of the cargo (in the eater).
• ESCRT in Development: It establishes a role for the ESCRT complex in developmental trogocytosis, extending its known functions in cytokinesis and viral budding.
• Therapeutic Implications: Understanding the distinct pathways in biting vs. bitten cells offers new targets for modulating trogocytosis in contexts like immune evasion (cancer cells stealing antigens) or immune activation (T-cell interactions).

Conclusion

The study concludes that RAB-35 is a master regulator of trogocytosis that operates via two distinct mechanisms: it drives digestion in the biting cell by regulating PIP2 turnover and phagosome maturation, and it drives scission in the bitten cell by cooperating with the ESCRT complex to sever the membrane. This work fundamentally shifts the understanding of trogocytosis from a one-sided "eating" event to a coordinated, two-cell interaction.