FHOD3 and DIAPH3 control cell migration and differentially shift the balance of parallel and perpendicular stress fibers

This study demonstrates that the formins FHOD3 and DIAPH3 differentially regulate distinct stress fiber networks—controlling the balance between perpendicular and parallel orientations—to govern cell morphology and migration speed.

The Gist

Imagine a cell as a tiny, busy construction crew trying to move across a landscape. To move, this crew needs a skeleton made of tiny ropes called actin fibers (specifically, "stress fibers"). These ropes pull the cell forward, help it change shape, and anchor it to the ground.

The ground they are walking on is the Extracellular Matrix (ECM). Sometimes the ground is soft like a marshmallow (soft tissue), and sometimes it's hard like a rock (stiff tissue).

This paper is about two specific "foremen" inside the cell construction crew: FHOD3 and DIAPH3. These foremen are proteins that help build and organize the actin ropes. The researchers wanted to know: Do these two foremen do the same job, or do they have different specialties?

Here is the story of what they found, explained simply:

1. The Terrain Matters (Stiffness)

First, the researchers noticed that cells react differently depending on how hard the ground is.

• The "Stretchy" Cell (HFF): When this cell walks on hard ground, it stretches out long and thin, like a person running on a track. It builds strong, long ropes to pull itself forward.
• The "Round" Cell (MDA-MB-231): This cell is a bit different. On soft ground, it stays long and thin. On hard ground, it actually gets rounder and shorter.
• The "Chill" Cell (HaCaT): This one doesn't care much about the ground; it stays roughly the same shape everywhere.

2. The Two Foremen (FHOD3 and DIAPH3)

The researchers looked at the "blueprints" (genes) of many different cells and found that FHOD3 and DIAPH3 were the ones most closely linked to how long and thin a cell gets. They decided to test what happens if they fire these foremen (knock them down) to see what goes wrong.

They discovered that these two foremen are specialists, not generalists. They build different types of ropes:

• FHOD3 is the "Longitudinal" Specialist: It loves building ropes that run parallel to the direction the cell wants to move (like the cables on a suspension bridge running straight down the road).
• DIAPH3 is the "Transverse" Specialist: It loves building ropes that run perpendicular (sideways) to the direction of movement (like the cross-beams that keep the bridge from collapsing inward).

3. The Great Balancing Act

The most exciting discovery is how these two foremen balance each other out, but it depends on the type of cell:

In the "Contractile" Cell (HFF - the one that pulls hard):

• If you fire FHOD3: The cell loses its parallel ropes. Suddenly, the DIAPH3 foreman takes over and builds too many sideways ropes. The cell gets confused, loses its long shape, and can't pull itself forward efficiently.
• If you fire DIAPH3: The cell loses its sideways ropes. The FHOD3 foreman goes wild and builds too many parallel ropes. The cell gets too stretched out and rigid.
• The Metaphor: Imagine a tug-of-war team. FHOD3 pulls the team forward. DIAPH3 keeps the team tight and together. If you remove the forward puller, the team gets pulled sideways. If you remove the tightener, the team stretches out too thin and falls apart.

In the "Motile" Cell (MDA-MB-231 - the fast runner):

• This cell is already very good at moving fast. If you fire either foreman, the whole construction crew slows down. Both types of ropes are needed for this cell to run at top speed. It's like a race car needing both a strong engine (parallel ropes) and good suspension (sideways ropes) to win.

4. Why Does This Matter?

Think of a cell trying to migrate through your body (like a healing cell closing a wound or a cancer cell spreading).

• To move in a straight line, the cell needs FHOD3 to build the "highway" ropes.
• To keep its shape and grip the ground, it needs DIAPH3 to build the "stabilizer" ropes.

If these two foremen get out of balance, the cell might get stuck, move in circles, or lose its shape. This is crucial for understanding how wounds heal and how cancer spreads.

The Bottom Line

This paper tells us that cells don't just build a random mess of ropes. They have a sophisticated tension balance system.

• FHOD3 pushes the cell to be long and directional.
• DIAPH3 keeps the cell stable and compact.

By tweaking the balance between these two, the cell can decide: "Do I need to stretch out to reach something?" or "Do I need to stay compact to squeeze through a tight space?" It's a delicate dance of construction, and these two foremen are the ones holding the keys.

Technical Summary

1. Problem Statement

Cell migration is fundamental to physiological processes (e.g., wound healing) and pathological events (e.g., cancer metastasis). This process is governed by the filamentous actin (F-actin) cytoskeleton, which responds to mechanical cues from the extracellular matrix (ECM), particularly substrate stiffness. While it is known that formins are key regulators of linear F-actin assembly into stress fibers, the specific roles of individual formin isoforms in controlling distinct subpopulations of stress fibers (parallel vs. perpendicular to the cell's long axis) and how these distinct networks differentially regulate cell morphology and migration across various cell types remain poorly defined.

2. Methodology

The study employed a multi-faceted approach combining cell biology, biophysics, and computational modeling:

• Cell Models: The researchers utilized a panel of five human cell lines representing diverse migration modes: mesenchymal (HFF, MDA-MB-231), epithelial (HaCaT), amoeboid (THP-1), and a migration-switching line (WM266-4).
• Substrate Engineering: Collagen-coated polyacrylamide (PAA) hydrogels with defined elastic moduli (0.2 kPa, 2 kPa, and 20 kPa) and stiff glass controls were fabricated to simulate varying ECM stiffness.
• Morphological & Migration Analysis:
  • Cell area and aspect ratio were quantified via fluorescence microscopy (phalloidin staining).
  • Cell migration speed and persistence were measured using time-lapse phase-contrast microscopy and tracked via the M-Tracker J plugin in ImageJ.
• Cytoskeletal Architecture: Stress fibers were analyzed for density, length, and orientation (classified as parallel <30°, transitional 30–60°, or perpendicular >60° relative to the cell's long axis) using local background subtraction algorithms.
• Genetic Screening & Knockdown:
  • Transcript levels of 15 formin isoforms were measured across cell lines using RT-PCR and RT-qPCR.
  • Partial Least Squares (PLS) Regression with Variable Importance in Projection (VIP) scoring was used to correlate formin expression with morphological features (area and aspect ratio).
  • siRNA-mediated knockdown (KD) of the top candidates, FHOD3 and DIAPH3, was performed in HFF and MDA-MB-231 cells to assess functional roles.

3. Key Contributions

• Identification of Cell-Type Specific Stiffness Responses: The study demonstrates that the morphological response to ECM stiffness is not universal; different cell types exhibit distinct phenotypic classes (stiffness-insensitive, positive correlation between stiffness and elongation, or negative correlation).
• Formin-Morphology Correlation: Through multivariate analysis, the authors identified FHOD3 and DIAPH3 as the formins most strongly correlated with cell aspect ratio (elongation), distinguishing them from other formins (like DIAPH1 and INF2) that correlate more with cell spreading area.
• Mechanistic Differentiation of Stress Fiber Populations: The paper establishes that FHOD3 and DIAPH3 do not merely regulate general F-actin assembly but specifically tune the balance between parallel and perpendicular stress fibers.
• Context-Dependent Migration Regulation: The study reveals that the requirement for these formins in migration speed is cell-type dependent, being critical for highly motile cells but less so for less motile, highly contractile cells.

4. Key Results

A. Morphology and Migration vs. Stiffness

• HFFs (Fibroblasts): Showed a strong positive correlation between substrate stiffness and both cell area and aspect ratio. They formed robust, aligned stress fibers only on stiff substrates.
• MDA-MB-231 (Breast Cancer): Exhibited a complex phenotype; they were highly elongated on soft substrates but became more rounded on stiff substrates. They migrated faster on stiff substrates but with lower persistence.
• General Finding: Aspect ratio correlated with migration persistence, but cell area did not correlate with migration speed.

B. Formin Expression and Function

• PLS Analysis: FHOD3 and DIAPH3 were identified as the primary drivers of cell elongation (aspect ratio).
• Knockdown Effects on Morphology:
  • In HFFs (on soft substrates), FHOD3 KD increased cell area and aspect ratio, while DIAPH3 KD decreased both.
  • In MDA-MB-231s, KD of either formin generally reduced cell area and aspect ratio.

C. Stress Fiber Architecture and Balance

• FHOD3 KD: Reduced the total length of parallel stress fibers. In contractile HFFs, this led to a shift toward perpendicular stress fibers (increasing the perpendicular/parallel ratio).
• DIAPH3 KD: Reduced the total length of perpendicular stress fibers. In contractile HFFs, this led to a shift toward parallel stress fibers (increasing the parallel/perpendicular ratio).
• MDA-MB-231s: While both KDs reduced total stress fiber length, they did not significantly alter the parallel/perpendicular balance, suggesting these cells have a different baseline cytoskeletal organization.

D. Migration Impact

• MDA-MB-231s: KD of either FHOD3 or DIAPH3 significantly reduced migration speed on both soft and stiff substrates, indicating both parallel and perpendicular networks are essential for high-speed motility in these cells.
• HFFs: KD of either formin had no significant effect on migration speed, despite altering stress fiber architecture and morphology. This suggests that in less motile, highly contractile cells, the balance of stress fibers is more critical for shape maintenance than for driving speed.

5. Significance

This work provides a refined model of cytoskeletal regulation where specific formin isoforms act as "tuners" of mechanical output:

1. FHOD3 primarily promotes parallel stress fibers (ventral stress fibers), which are associated with axial traction forces and contact guidance.
2. DIAPH3 primarily promotes perpendicular stress fibers (transverse arcs), which generate radial tension and help maintain cell shape/thinness.
3. Differential Mechanisms: The study highlights that the functional outcome of formin depletion depends heavily on the cell's intrinsic contractility and migration mode. In highly motile cells, both networks are required for speed; in contractile cells, the formins act antagonistically to balance axial vs. radial forces, thereby controlling cell shape and ECM remodeling capabilities.

These findings offer new insights into how cancer cells might manipulate their cytoskeleton to navigate different tissue stiffnesses and suggest potential targets for modulating cell migration in therapeutic contexts.