SPIN90 modulates the architecture of lamellipodial actin in an ARPC5L dependent fashion.

This study demonstrates that SPIN90 selectively activates ArpC5L-containing Arp2/3 complexes to generate linear actin filaments, thereby regulating the architecture and protrusion efficiency of lamellipodia in migrating cells.

The Gist

Imagine your cell is a bustling construction site, constantly building and rebuilding its outer walls to move forward. The "bricks" it uses are tiny protein chains called actin filaments. To build these walls quickly and efficiently, the cell needs a specialized construction crew: the Arp2/3 complex.

Think of the Arp2/3 complex as a master branching machine. Usually, when it gets a signal from a foreman named WAVE, it grabs an existing actin chain and sprouts a new one at a sharp 70-degree angle. This creates a dense, tree-like network (a "branched" network) that pushes the cell membrane forward, like a crowd of people pushing a door open.

However, this paper introduces a different foreman named SPIN90. When SPIN90 calls the shots, the Arp2/3 machine doesn't make branches; instead, it lays down straight, linear lines of bricks.

The Big Discovery: A "Right-Hand" vs. "Left-Hand" Problem

Here is the twist: The Arp2/3 machine isn't just one single tool. In mammals, it comes in two slightly different versions, like a pair of gloves: a Left Glove and a Right Glove.

• The "Right Glove" (ArpC5): This version is the standard worker.
• The "Left Glove" (ArpC5L): This is a special variant.

The researchers discovered that SPIN90 is picky. It only knows how to talk to and activate the Left Glove (ArpC5L) version of the machine. It completely ignores the Right Glove (ArpC5).

• Analogy: Imagine a construction site with two types of cranes. SPIN90 is a specific remote control that only works on the "Left-Hand" cranes. If you try to use it on a "Right-Hand" crane, nothing happens.

What Happens at the Cell's Edge?

When a cell wants to move (migrate), it extends a "foot" called a lamellipodium. This is the leading edge where all the construction happens.

1. Recruitment: SPIN90 gathers at the very front of this foot. Because it only talks to the "Left Glove" (ArpC5L) machines, it pulls those specific machines to the front line.
2. Building: These activated machines start laying down straight, linear actin lines.
3. The Result: These straight lines weave into the existing branched network. This changes the architecture of the wall. Instead of just a dense, rigid forest of branches, you now have a mix of branches and straight lines running in different directions.

Why Does This Matter? (The "Traffic Flow" Analogy)

The researchers used a special type of "polarized light" camera to see the direction of these actin bricks.

• Without SPIN90 (or without the Left Glove): The actin network becomes too uniform. It's like a traffic jam where all cars are driving in the exact same direction, perpendicular to the road. It's rigid and doesn't flow well. The cell's "foot" becomes stiff and moves slowly.
• With SPIN90: The straight lines act like diversifying traffic lanes. They allow the actin network to have a wider variety of angles. This flexibility makes the "foot" more efficient at pushing forward.

The Finding: Cells that lack SPIN90 (or the specific "Left Glove" machine) move much slower. They have a "stiff" foot that can't push efficiently because the construction is too rigid and lacks the necessary variety in direction.

The "Rearward Flow" Mystery

The paper also observed something cool: The SPIN90 protein and the machines it activates don't just sit still. They get built into the wall and then flow backward toward the center of the cell (like a conveyor belt moving backward).

• Analogy: Imagine a conveyor belt at a factory. The bricks are laid down at the front, but the whole belt moves backward. The researchers found that SPIN90 rides this belt only if it is holding hands with the "Left Glove" machine. If SPIN90 is broken and can't hold hands, it just sits at the front and doesn't move. This proves that SPIN90 and the specific machine are a team that gets integrated into the moving structure.

Summary in Plain English

1. The Team: The cell uses a machine (Arp2/3) to build its skeleton. This machine comes in two slightly different flavors.
2. The Boss: A protein called SPIN90 acts as a boss that tells the machine to build straight lines instead of branches.
3. The Catch: SPIN90 only works with one specific flavor of the machine (the one with the "ArpC5L" part). It ignores the other flavor.
4. The Consequence: By bringing in this specific flavor to build straight lines, SPIN90 makes the cell's "foot" more flexible and efficient. Without it, the foot becomes too rigid, and the cell moves slowly.

In short: SPIN90 is the specialized foreman that ensures the cell builds a flexible, multi-directional skeleton, allowing it to move quickly and efficiently through the body. Without this specific foreman, the construction crew gets stuck in a rigid pattern, and the cell loses its stride.

Technical Summary

1. Problem Statement

The Arp2/3 complex is a conserved actin nucleator essential for generating branched actin networks, which drive cellular processes like migration and endocytosis. While typically activated by nucleation-promoting factors (NPFs) like WAVE to create branched networks, the Arp2/3 complex can also be activated by SPIN90 (also known as WISH/NCKIPSD) to generate linear actin filaments.

In mammals, the Arp2/3 complex exists as multiple iso-complexes due to the presence of two isoforms for three of its subunits: Arp3, ArpC1, and ArpC5 (ArpC5 and ArpC5L). Previous studies suggested these iso-complexes have distinct properties, but it remained unclear:

• Whether all Arp2/3 iso-complexes can generate linear filaments when activated by SPIN90.
• How the specific subunit composition (specifically ArpC5 vs. ArpC5L) influences SPIN90-mediated activation.
• The physiological impact of this selectivity on the architecture and dynamics of the actin cytoskeleton in migrating cells.

2. Methodology

The authors employed a multi-faceted approach combining in vitro biochemistry, structural biology, and advanced live-cell imaging:

• Recombinant Protein Production: Purified human Arp2/3 iso-complexes with defined compositions (varying ArpC1 and ArpC5 isoforms) and SPIN90.
• In Vitro Biochemical Assays:
  • Pyrene-actin polymerization assays: To measure the rate of actin nucleation and polymerization.
  • TIRF Microscopy: To directly visualize and quantify the generation of linear actin filaments by different iso-complexes.
  • GST Pull-down Assays: To determine the binding affinity and dissociation kinetics between SPIN90 and various Arp2/3 iso-complexes.
• Structural Analysis: Utilized existing cryo-EM structures (PDB: 6DEC) and sequence alignments to identify interaction interfaces between SPIN90 and ArpC5/ArpC5L.
• Cell Biology (B16 Melanoma Cells):
  • CRISPR-Cas9 Genome Editing: Generated knockout (KO) cell lines for ArpC5, ArpC5L, and SPIN90.
  • Endogenous Tagging: Used the ORANGE toolbox to fuse NeonGreen to endogenous ArpC5 or ArpC5L, and GFP to $\beta$-actin.
  • siRNA Depletion: Used to knock down specific isoforms to study compensatory mechanisms.
  • Live Cell Imaging: Used spinning disk confocal and VT-iSIM (instant Structured Illumination Microscopy) to track protein localization and retrograde flow.
  • Polarized Fluorescence Microscopy: A specialized technique using Alexa-488 phalloidin to measure the mean orientation ($\rho$) and alignment ($\psi$) of actin filaments at the molecular scale (~200 nm resolution).
  • FRAP (Fluorescence Recovery After Photobleaching): To quantify lamellipodial protrusion rates, rearward flow, and actin polymerization efficiency.

3. Key Contributions and Results

A. Selective Activation of ArpC5L-containing Complexes

• Specificity: In vitro assays revealed that SPIN90 selectively activates Arp2/3 complexes containing the ArpC5L isoform to generate linear actin filaments. Complexes containing the ArpC5 isoform (regardless of the ArpC1 isoform) were poorly activated by SPIN90 and failed to generate significant linear filaments.
• Binding Affinity: GST pull-down assays demonstrated that SPIN90 has a significantly higher affinity for ArpC5L-containing complexes ($K_D \approx 3.01 \, \mu M$) compared to ArpC5-containing complexes. The dissociation rates were similar, but the binding capacity was drastically different, suggesting positive cooperativity for the ArpC5L complex.
• Structural Basis: While existing structures lacked side-chain resolution for the ArpC5 N-terminus, sequence alignment and biochemical data suggest that specific residues in the ArpC5L isoform are critical for the high-affinity interaction with SPIN90.

B. Cellular Localization and Recruitment

• Leading Edge Enrichment: In migrating B16 cells, SPIN90 accumulates at the leading edge of lamellipodia.
• Selective Recruitment: SPIN90 enhances the recruitment of ArpC5L-containing Arp2/3 complexes to the leading edge. Depletion of SPIN90 reduces ArpC5L levels at the edge, whereas ArpC5 levels are unaffected or slightly increased (suggesting a compensatory mechanism).
• Retrograde Flow: Live imaging showed that full-length SPIN90 undergoes retrograde flow (moving rearward from the leading edge) in concert with ArpC5L. However, a truncated SPIN90 construct (lacking the Arp2/3 binding domain) remained static at the leading edge. This confirms that SPIN90 integrates into the actin network via its interaction with ArpC5L-containing complexes.

C. Regulation of Actin Architecture

• Filament Orientation: Using polarized fluorescence microscopy, the authors quantified the orientation of actin filaments.
  • Wild Type (WT): Filaments showed a broad distribution of angles.
  • ArpC5L KO / SPIN90 KO: The distribution of filament angles became narrower, with a significant enrichment of filaments oriented perpendicular to the plasma membrane. This indicates a loss of the diverse, linear filaments normally nucleated by SPIN90-ArpC5L.
  • ArpC5 KO: Filament orientation distribution was broader than WT, likely due to a compensatory increase in ArpC5L activity and potential interactions with Ena/VASP.
• Mechanism: The authors propose that while Arp2/3-mediated branches are geometrically constrained to ~70°, SPIN90-nucleated linear filaments can orient in various directions before being crosslinked, thereby increasing the diversity of filament angles at the leading edge.

D. Impact on Cell Migration and Protrusion

• Protrusion Efficiency:
  • SPIN90 KO: Cells exhibited a reduced protrusion rate and lower protrusion efficiency (protrusion rate / polymerization rate), despite having similar lamellipodial width to WT.
  • ArpC5L KO: Surprisingly, these cells maintained a protrusion rate comparable to WT, likely due to the upregulation of ArpC5-mediated branching and Ena/VASP activity.
  • ArpC5 KO: Cells showed reduced protrusion rates and migration speeds.
• Migration Speed: Both SPIN90 KO and ArpC5 KO cells migrated significantly slower than WT cells in random migration assays.
• Overexpression: Overexpressing full-length SPIN90 increased the lamellipodial protrusion rate, confirming its role in promoting efficient protrusion.

4. Significance

This study provides a definitive mechanistic link between Arp2/3 subunit isoforms and specific cellular functions:

1. Iso-complex Specificity: It establishes SPIN90 as the first NPF with a clear preference for a specific subset of Arp2/3 iso-complexes (ArpC5L-containing), distinguishing the roles of ArpC5 and ArpC5L.
2. Architecture Control: It demonstrates that the selective recruitment of ArpC5L by SPIN90 is crucial for generating a diverse actin filament architecture (mixing linear and branched filaments) at the leading edge.
3. Functional Consequence: The loss of this specific linear filament population (in SPIN90 or ArpC5L KO) leads to a "stiffening" of the actin network (more perpendicular filaments), which reduces the efficiency of lamellipodial protrusion and cell migration.
4. Disease Relevance: Given that mutations in ArpC5 and ArpC5L are linked to severe immunodeficiencies and inflammation, understanding their distinct roles in cytoskeletal dynamics offers new insights into the pathophysiology of these conditions.

In summary, the paper elucidates how SPIN90 acts as a molecular switch that selectively engages ArpC5L-containing Arp2/3 complexes to generate linear actin filaments, thereby tuning the architecture and dynamics of the lamellipodium to optimize cell motility.