Galectin-1 Modulates Cell Adhesions, Caveolae, and Vascular Permeability in Kidney Endothelial Cells -- Insights from Proteomics, Phosphoproteomics, and Functional Studies

This study demonstrates that galectin-1 modulates glomerular endothelial cell adhesion, caveolae formation, and vascular permeability through specific proteomic and phosphoproteomic changes, identifying it as a critical therapeutic target for antibody-mediated kidney rejection.

The Gist

The Big Picture: A Leaky Filter in the Kidney

Imagine your kidney is a high-tech coffee filter. Its job is to let water and waste pass through while keeping the good stuff (like proteins and blood cells) inside. This filter is lined with a delicate layer of cells called endothelial cells.

In people who have received a kidney transplant, the body sometimes attacks this new filter. This is called Antibody-Mediated Rejection (ABMR). When this happens, the "coffee filter" gets damaged, becomes leaky, and starts letting good stuff escape. This leads to kidney failure.

Scientists knew that a specific inflammatory signal, called IFNγ (think of it as a "Siren Alarm" sent by the immune system), causes this damage. But they didn't fully understand how the damage happened or what specific proteins were involved in the breakdown.

This study focuses on a protein called Galectin-1. Think of Galectin-1 as the construction foreman or the glue that helps the endothelial cells stick together and hold their shape. The researchers wanted to know: What happens to the kidney filter if we mess with this foreman while the "Siren Alarm" is blaring?

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The Experiment: A High-Tech Simulation

To figure this out, the scientists didn't just look at old kidney samples; they built a miniature, 3D kidney filter in a lab dish (a microfluidic chip). They grew human kidney cells inside tiny tubes to mimic real blood vessels.

They ran four different scenarios:

1. Normal cells: Just chilling.
2. Alarm only: They turned on the "Siren Alarm" (IFNγ) to simulate inflammation.
3. Foreman fired: They removed the "construction foreman" (Galectin-1) from the cells.
4. Alarm + Foreman fired: They turned on the alarm and fired the foreman at the same time.

They then used a super-powerful microscope (Proteomics) to take a snapshot of every single protein inside the cells to see how they changed.

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Key Findings: What Happened?

1. Who is the Foreman?

First, they confirmed that in damaged kidneys, the endothelial cells (the filter lining) are the main source of Galectin-1. It's like realizing the construction foreman lives right inside the building he's supposed to protect.

2. The "Glue" Breaks Down

When they removed Galectin-1 (fired the foreman) while the alarm was ringing, the cells didn't just sit there. They started acting chaotic.

• The Analogy: Imagine a brick wall. Galectin-1 is the mortar holding the bricks together. When you remove the mortar while a storm (IFNγ) hits, the bricks don't just fall; they rearrange themselves into a weird, unstable pattern.
• The Science: The cells changed their "adhesion" proteins. They stopped sticking to their neighbors properly and started grabbing onto the wrong things. Specifically, a protein called ITGB5 (a type of anchor) went haywire.

3. The "Cave" Collapse

The cells have tiny, flask-shaped pockets on their surface called caveolae. Think of these as sensors and airlocks that help the cell feel pressure and move things in and out.

• The Analogy: The study found that when the foreman was gone, the "airlocks" (caveolae) stopped forming correctly. The sensors (proteins) that usually sit at the door of these airlocks fell off.
• The Result: Without these sensors, the cell loses its ability to sense its environment and maintain a tight seal.

4. The Leaky Filter

This is the most important part. They tested how "leaky" their mini-kidney filters became.

• Alarm alone: The filter got a little leaky.
• Foreman gone alone: The filter stayed mostly okay.
• Alarm + Foreman gone: Disaster. The filter became extremely leaky. Water and large particles (simulating blood proteins) poured through.
• The Twist: When they added extra Galectin-1 from the outside (like hiring a new foreman from the street), it actually made the leak worse in the damaged cells, but helped the healthy cells. This suggests that in a damaged kidney, the "foreman" is confused and needs to be fixed from the inside, not just patched from the outside.

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Why Does This Matter?

This study solves a mystery about why kidney transplants fail. It shows that the damage isn't just the immune system attacking; it's a specific chain reaction where:

1. Inflammation (IFNγ) triggers a crisis.
2. The cell's internal "foreman" (Galectin-1) gets confused or depleted.
3. The cell's "glue" and "sensors" break down.
4. The kidney filter becomes a sieve, letting vital proteins leak out.

The Takeaway:
Galectin-1 is a double-edged sword. Inside the cell, it helps keep the filter tight. But if the cell is already inflamed, messing with Galectin-1 (either by removing it or adding too much from the outside) can make the leak worse.

The Future:
This gives doctors a new target. Instead of just trying to stop the immune system (the "Siren Alarm"), we might be able to develop drugs that restore the "construction foreman" (Galectin-1) to its proper job, helping the kidney filter repair itself and stop leaking. This could save many more kidney transplants from failing.

Technical Summary

1. Problem Statement

Antibody-mediated rejection (ABMR) is the leading cause of premature kidney allograft loss. A hallmark of ABMR is microvascular inflammation and endothelial injury, driven by pro-inflammatory cytokines such as Interferon-gamma (IFN$\gamma$). While glomerular extracellular matrix (ECM) remodeling is a known feature of chronic ABMR, the specific molecular mechanisms linking endothelial dysfunction, ECM changes, and immunomodulatory proteins remain incompletely understood.

Previous observations indicated that galectin-1 (encoded by LGALS1), a carbohydrate-binding immunomodulatory protein, is increased in the glomeruli of ABMR patients, yet its gene expression is downregulated by IFN$\gamma$ in endothelial cells. The study aims to dissect how the modulation of galectin-1 in glomerular microvascular endothelial cells (GMECs), particularly in the context of inflammatory stress (IFN$\gamma$), affects endothelial barrier integrity, signaling pathways, and vascular permeability.

2. Methodology

The study employed a multi-omics approach combined with functional assays using primary human GMECs and a microfluidic model.

• Cell Models & Experimental Design:

  • Primary human GMECs were used.
  • Intervention: LGALS1 was knocked down (siRNA) or cells were treated with recombinant galectin-1 (r-galectin-1).
  • Stimulation: Cells were treated with IFN$\gamma$ (50 U/mL) to mimic the inflammatory environment of ABMR.
  • Conditions: Four groups were analyzed: (1) siRNA + IFN$\gamma$, (2) siRNA + Vehicle, (3) Control + IFN$\gamma$, (4) Control + Vehicle.
  • Timepoints: 30 minutes for phosphoproteomics (signaling) and 24 hours for proteomics (expression).

• Omics Profiling:

  • Label-free Quantitative Proteomics: LC-MS/MS analysis identified 5,446 proteins. Statistical analysis (2-way ANOVA) focused on differentially expressed proteins (FDR < 0.05).
  • Phosphoproteomics: TiO$_2$ enrichment followed by LC-MS/MS identified 2,727 phosphopeptides. Analysis used MSstatsPTM to identify significant phosphorylation changes (FDR < 0.2).
  • Bioinformatics: Pathway enrichment (GO, pathDIP), protein interaction networks (IID, NAViGaTOR), and clustering were performed.

• Validation & Functional Assays:

  • Imaging Mass Cytometry (IMC): Used on human ABMR biopsies to localize galectin-1 expression.
  • Flow Cytometry & Immunofluorescence: Validated galectin-1 expression in GMECs vs. PBMCs and assessed co-localization of Caveolin-1 (CAV1) and Caveolae-associated protein 1 (CAVN1).
  • Microfluidic AngioPlate Model: A 3D perfusable glomerular microvascular model was used to assess:
    • Barrier Integrity: Measured via permeability of 4 kDa and 65 kDa FITC/TRITC-dextran.
    • Cytokine Secretion: Flow-through analysis using a 15-plex pro-inflammatory cytokine assay.
    • Structural Markers: Immunostaining for CD31, VE-cadherin, and galectin-1.

3. Key Contributions

• Cellular Source Identification: Established GMECs as the predominant source of galectin-1 in the kidney glomerulus during ABMR, rather than immune cells.
• Multi-Omic Mechanism: Defined the specific proteomic and phosphoproteomic signatures of galectin-1 depletion under inflammatory stress, linking LGALS1 loss to cytoskeletal and adhesion dysregulation.
• Functional Duality: Demonstrated that the effect of galectin-1 on vascular permeability is context-dependent: intracellular depletion exacerbates injury, while extracellular addition has divergent effects based on the presence of inflammation or endogenous depletion.
• Novel Platform Application: Successfully utilized a 3D microfluidic glomerular model to translate molecular findings into functional vascular permeability data.

4. Key Results

A. Cellular Localization

• Single-cell RNA sequencing and flow cytometry confirmed that GMECs are the primary source of galectin-1 in the kidney, with minimal expression in peripheral blood mononuclear cells (PBMCs).
• IMC of ABMR biopsies showed strong co-localization of galectin-1 with CD31+ endothelial cells, but not CD45+ immune cells.

B. Proteomic Changes (LGALS1 Knockdown)

• LGALS1 Knockdown alone: Increased expression of ECM and focal adhesion proteins (e.g., CD44, ITGB1, ITGB5, PECAM1) and decreased expression of IFN-associated immune proteins. This suggests endogenous galectin-1 normally suppresses adhesion/matrix programs to balance inflammatory signaling.
• IFN$\gamma$ Treatment: Upregulated immune signaling pathways and downregulated ECM organization.
• Interaction (Knockdown + IFN$\gamma$): 267 proteins showed significant interaction effects. Key targets included ITGB5 (Integrin $\beta$5), PLAU, and cytoskeletal regulators. Validation via Western blot confirmed ITGB5 upregulation.

C. Phosphoproteomic Changes

• CAVN1 Phosphorylation: A specific phosphopeptide of Caveolae-associated protein 1 (CAVN1) was significantly decreased by IFN$\gamma$, LGALS1 knockdown, and their combination.
• Caveolae Disruption: Reduced CAVN1 phosphorylation led to diminished co-localization of CAVN1 with CAV1 (Caveolin-1), indicating disrupted caveolar assembly. This is critical as caveolae are essential for mechanosensing and endothelial barrier function.

D. Functional Outcomes (AngioPlate Model)

• Permeability:
  • IFN$\gamma$ alone increased permeability (loss of barrier integrity).
  • LGALS1 knockdown alone had no effect on baseline permeability.
  • Synergy: The combination of LGALS1 knockdown + IFN$\gamma$ significantly increased permeability to 4 kDa dextran.
  • Exogenous r-galectin-1:
    • In baseline conditions (no IFN$\gamma$/no knockdown), r-galectin-1 decreased permeability (stabilizing the barrier).
    • In the presence of IFN$\gamma$ or LGALS1 knockdown, r-galectin-1 increased permeability, exacerbating the injury.
• Cytokine Secretion:
  • GM-CSF: Levels decreased significantly with IFN$\gamma$, LGALS1 knockdown, or r-galectin-1 addition. Since GM-CSF supports angiogenesis and barrier stability, its loss contributes to dysfunction.
  • IL-6 & Pro-inflammatory Cytokines: IFN$\gamma$ increased IL-6. Interestingly, exogenous r-galectin-1 further amplified IL-6, IL-8, and MCP-1 secretion, suggesting a pro-inflammatory role for extracellular galectin-1 in this context.

5. Significance and Conclusion

This study elucidates a complex, context-dependent role for galectin-1 in kidney endothelial health:

1. Molecular Switch: Endogenous galectin-1 acts as a regulator balancing inflammatory signaling (IFN$\gamma$) with structural adhesion and ECM integrity. Its depletion shifts the endothelial phenotype toward increased adhesion molecule expression but disrupted junctional stability (VE-cadherin loss) and caveolar dysfunction.
2. Therapeutic Implications: The findings suggest that galectin-1 biology in ABMR is not linear. While intracellular galectin-1 supports barrier function (via GM-CSF and caveolae), extracellular galectin-1 in an inflamed environment may act as a damage-associated molecular pattern (DAMP), driving further inflammation and permeability.
3. Future Directions: Targeting galectin-1 pathways or restoring downstream endothelial repair mechanisms (e.g., caveolae assembly, GM-CSF signaling) could offer novel therapeutic strategies to mitigate chronic glomerular injury in kidney transplantation.

The study highlights the necessity of distinguishing between intracellular and extracellular galectin-1 functions when designing therapies for microvascular diseases.