CaRPOOL: A Pooled Calcium-Recording CRISPR Screening Platform Identifies CCR7 as a Modulator of Cellular Osmomechanosensing

This study introduces CaRPOOL, a high-throughput CRISPRi screening platform that integrates CaMPARI2 to capture transient calcium signals, enabling the discovery of CCR7 as a novel GPCR that modulates cellular osmomechanosensing through a PIEZO1-dependent pathway.

The Gist

The Big Idea: Catching a "Flash" in a Bucket

Imagine your cells are like tiny cities. Every day, these cities get sudden "alarms" from the outside world—like a sudden push (mechanical force) or a change in water pressure (osmotic stress). When the alarm rings, the city lights up with a burst of energy (calcium signals) to tell the inhabitants what to do.

The Problem: These bursts of light are incredibly fast. They happen in seconds and then vanish. Trying to study them is like trying to count how many fireflies flashed in a forest by standing there with a stopwatch. You can only watch a few at a time, and if you miss the flash, it's gone forever. This makes it very hard to find out which specific parts of the cell are responsible for turning on the lights.

The Solution: CaRPOOL
The researchers built a new tool called CaRPOOL (Calcium-Recording Pooled screening platform). Think of this as a special camera that doesn't just take a photo; it takes a "permanent snapshot" of the light.

1. The Camera (CaMPARI2): They used a special protein called CaMPARI2. Imagine this protein is a chameleon that is normally green. But, if it sees a flash of calcium light while a violet flashlight is shining on it, it permanently turns red.
2. The Process: They shine the violet light on a huge crowd of cells while they give them a "push" (osmotic stress).
   • Cells that reacted strongly to the push turned bright red.
   • Cells that didn't react stayed green.
3. The Sorting: They used a high-speed sorter (like a bouncer at a club) to separate the red cells from the green ones.
4. The Detective Work: They looked at the "green" cells (the ones that didn't react) to see what was missing. By using a genetic "scissors" tool (CRISPR) to turn off different genes one by one, they could figure out which gene was the "switch" needed to turn on the calcium alarm.

The Discovery: The "Secret Agent" Receptor

Using this new camera system, they found a surprise suspect: a protein called CCR7.

• Who is CCR7? Usually, CCR7 is known as a "traffic cop" for immune cells. It tells white blood cells where to go when there is an infection, based on chemical signals (like a scent trail).
• The Twist: The researchers found that CCR7 also acts as a mechanical sensor. It can feel when the cell is being squished or stretched by water pressure, even without any chemical scent trail.
• The Analogy: Imagine CCR7 is a security guard who usually only opens the gate when he smells smoke (chemical signal). But the researchers found out that if the building shakes violently (mechanical stress), this guard opens the gate anyway, even if there's no smoke.

How It Works: The Domino Effect

Once CCR7 feels the "shake," it starts a chain reaction inside the cell, like a row of falling dominoes:

1. The Trigger: CCR7 gets bumped by the mechanical stress.
2. The Messenger: It taps a messenger (Gαs) who runs to a factory (Adenylate Cyclase).
3. The Fuel: The factory produces a fuel called cAMP.
4. The Spark: This fuel powers an engine (PKA) that goes straight to the cell's main "water valve" (a protein called PIEZO1).
5. The Result: The engine sprays the valve with oil (phosphorylation), making it super sensitive. Suddenly, the valve opens wide, letting a flood of calcium rush in to sound the alarm.

Why Does This Matter? (The Immune System Connection)

This discovery is huge for understanding our immune system.

• The Scenario: Imagine your immune cells are soldiers marching through a crowded, narrow tunnel (your blood vessels and tissues). They are constantly getting squeezed, pushed, and stretched by the environment.
• The Feedback Loop: The paper found that when these immune cells get squeezed, they actually make more CCR7.
  • Analogy: It's like a soldier getting hit by a rock, realizing they need better armor, and immediately building more armor on the spot.
• The Result: The more they get squeezed, the more sensitive they become to the next squeeze. This helps them adapt quickly to the rough-and-tumble environment of the body, allowing them to react faster to threats.

Summary

The team invented a "permanent camera" (CaRPOOL) to catch fleeting cell signals. They used it to discover that a traffic cop (CCR7) also doubles as a shock absorber. When immune cells get squeezed, this protein wakes up, turns on a specific chain of events, and makes the cells super-sensitive to future bumps, helping our immune system adapt to the physical stresses of life inside our bodies.

Technical Summary

1. The Problem

Cellular adaptation to environmental stimuli (mechanical, thermal, chemical) relies on the rapid transduction of physical cues into intracellular signals, often involving transient calcium ($Ca^{2+}$) spikes. Identifying the genes governing these sensory processes has historically been limited by two major bottlenecks:

• Transient Nature of Signals: Calcium signaling events occur within seconds to minutes, making them difficult to capture in pooled screening formats where cells are analyzed simultaneously.
• Low Throughput: Traditional methods (e.g., patch-clamp electrophysiology or real-time calcium imaging) require individual cell observation, preventing genome-scale or large-scale pooled genetic screens.
• Lack of Scalable Tools: Existing genetic screens for sensory biology often rely on specialized, low-throughput instrumentation or static readouts (like transcriptional reporters) that miss dynamic signaling events.

2. Methodology: The CaRPOOL Platform

The authors developed CaRPOOL (Calcium-Recording Pooled screening platform), a high-throughput system that integrates three key technologies:

• CaMPARI2 (Calcium-Activated Photoactivatable Ratiometric Integrator): A genetically encoded fluorescent protein that irreversibly switches from green to red fluorescence upon exposure to violet light only in the presence of elevated intracellular calcium. This converts a transient calcium spike into a stable, cumulative fluorescence ratio (Red/Green) that can be measured at a single-cell level via flow cytometry.
• CRISPR Interference (CRISPRi): A pooled library of single-guide RNAs (sgRNAs) targeting 2,418 membrane-associated genes (including GPCRs and ion channels) was delivered into cells expressing a catalytically dead Cas9 (dCas9)-KRAB repressor. This allows for the systematic silencing of target genes.
• Osmomechanical Stimulation: Instead of complex physical devices, the authors used hypotonic stress (diluting culture medium with water) to induce cell swelling and membrane deformation, triggering mechanosensitive calcium influx in a scalable, uniform manner.
• Pooled FACS Screening:
  1. Cells were subjected to hypotonic stress while simultaneously illuminated with violet light to photoconvert CaMPARI2.
  2. Cells were sorted via Fluorescence-Activated Cell Sorting (FACS) based on the Red/Green ratio.
  3. The "bottom 15%" of cells (lowest calcium response) were isolated.
  4. sgRNA abundance in the sorted population vs. the input population was quantified via Next-Generation Sequencing (NGS) to identify genes whose knockdown reduced calcium signaling.

3. Key Contributions

• Platform Development: Established CaRPOOL as a broadly applicable, high-throughput framework for discovering regulators of dynamic calcium signaling, overcoming the temporal limitations of traditional imaging.
• Novel Gene Discovery: Identified CCR7 (a chemokine receptor) as a previously uncharacterized modulator of osmomechanosensing.
• Mechanistic Elucidation: Defined a non-canonical signaling axis where CCR7 modulates mechanotransduction independently of its ligands (CCL19/CCL21).
• Immune Adaptation Insight: Revealed a positive feedback loop in immune cells where mechanical stress upregulates CCR7, which in turn sensitizes cells to further mechanical cues.

4. Key Results

A. Validation of CaRPOOL for Osmomechanosensing

• HEK293T cells expressing CaMPARI2 showed robust, osmolarity-dependent Red/Green ratio increases upon hypotonic and hypertonic stimulation.
• Hypotonic stimulation (1:1 water-to-medium dilution) elicited the strongest response and was selected for screening.
• The response was confirmed to be driven by osmotic pressure rather than ionic composition.

B. CRISPRi Screen and Hit Identification

• Screening the membrane-associated library identified known mechanotransducers (e.g., TMC1, USH2A, PIEZO1, ADGRG6) as validation hits.
• Top Novel Hit: CCR7 showed the most significant reduction in calcium response upon knockdown.
• Validation:
  • Knockdown of CCR7 significantly reduced hypotonic-induced calcium influx.
  • Overexpression of CCR7 enhanced calcium influx.
  • Overexpression of the related receptor CCR6 had no effect, confirming specificity.
  • The effect was ligand-independent; mutations disrupting CCR7 binding to CCL21 did not impair its ability to enhance mechanosensing.

C. Mechanism of Action: CCR7-PIEZO1 Axis

• Source of Calcium: The response relies on extracellular calcium influx, not ER release (knockdown of ITPR3 had no effect; removal of extracellular $Ca^{2+}$ abolished the response).
• PIEZO1 Dependence: The effect of CCR7 is strictly dependent on the mechanosensitive ion channel PIEZO1. Overexpression of CCR7 failed to enhance responses in PIEZO1-knockout cells.
• Signaling Pathway: CCR7 modulates PIEZO1 via the G$\alpha$s–cAMP–PKA pathway:
  • Knockdown of GNAS (G$\alpha$s), ADCY3 (Adenylyl Cyclase), or PRKACA (PKA catalytic subunit) abolished the CCR7-mediated enhancement.
  • Hypotonic stimulation increased intracellular cAMP levels.
  • Inhibitors of AC (SQ22536) and PKA (H-89) blocked the response.
  • Conclusion: CCR7 engages G$\alpha$s to activate AC $\rightarrow$ cAMP $\rightarrow$ PKA, which likely phosphorylates and sensitizes PIEZO1.

D. Physiological Relevance in Immune Cells

• Transcriptomics: RNA-seq of Jurkat T-cells under hypotonic stress revealed upregulation of CCR7 and pathways related to T-cell activation.
• Feedback Loop: Hypotonic stress increased CCR7 protein levels on the surface of Jurkat and DC2.4 dendritic cells.
• Functional Impact: Overexpressing CCR7 in these immune cells further amplified calcium influx upon hypotonic stress.
• Significance: This suggests a mechanoadaptive positive feedback loop: mechanical stress induces CCR7 expression, which sensitizes the cell to subsequent mechanical stimuli, potentially aiding immune cell migration and adaptation in dynamic tissue environments.

5. Significance

• Technological Advancement: CaRPOOL provides a scalable solution for functional genomics of transient signaling events, applicable beyond mechanosensing to any stimulus that triggers calcium flux (e.g., GPCR activation, neuronal activity).
• Biological Insight: The discovery of CCR7 as a mechanically responsive GPCR expands the repertoire of mechanosensors beyond ion channels. It reveals a crosstalk mechanism where a GPCR (CCR7) regulates an ion channel (PIEZO1) via a second messenger pathway (cAMP/PKA).
• Immunological Implications: The findings suggest that immune cells utilize a self-reinforcing mechanosensory circuit to adapt to the physical stresses encountered during circulation and tissue infiltration, offering new perspectives on immune mechanobiology and potential therapeutic targets for inflammatory or autoimmune conditions.