The epidermal stem cell-supporting matricellular protein fibulin 7 modulates skin inflammatory response in a psoriasis model

Fibulin 7, a matricellular protein supporting epidermal stem cells, mitigates skin inflammation in a psoriasis model by directly binding to IL-17A to suppress its signaling, a protective mechanism that is compromised in human psoriatic lesions where Fibulin 7 levels are reduced.

The Gist

The Big Picture: A Skin Shield Against Inflammation

Imagine your skin is a bustling city. The epidermis (the top layer) is the neighborhood where the residents (skin cells) live and work. Deep in the foundation of this neighborhood, right at the border between the skin and the muscle below, lives a special group of "construction workers" called stem cells. These workers are responsible for repairing the city and keeping it growing healthy.

Usually, this neighborhood is peaceful. But sometimes, a storm hits—like an infection or an autoimmune reaction (in this study, they used a cream called Imiquimod to simulate a storm similar to Psoriasis). When the storm hits, the city goes into panic mode: the walls get thick, the workers start building too fast, and inflammation runs wild.

This paper asks a simple question: Is there a "safety net" in the foundation that protects these construction workers from the storm?

The answer is yes, and the name of that safety net is Fibulin 7.

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The Story of the Missing Safety Net

1. The Experiment: Removing the Shield

The scientists took two groups of mice:

• Group A (The Normal Ones): They have the safety net (Fibulin 7).
• Group B (The "KO" Mice): They are missing the safety net (Fibulin 7 is knocked out/removed).

They applied the "storm cream" (Imiquimod) to both groups.

• The Result: The mice with the safety net had a mild reaction. Their skin got a little red and dry, but it was manageable.
• The Disaster: The mice without the safety net had a much worse reaction. Their skin became incredibly thick, scaly, and inflamed. It was like the city walls were crumbling and rebuilding themselves chaotically.

The Lesson: Fibulin 7 acts like a shock absorber. Without it, the skin cells get hit too hard by the inflammatory storm and go into overdrive.

2. The Culprit: The "Angry Messenger" (IL-17A)

The scientists wanted to know why the mice without the safety net got so sick. They discovered the main villain was a chemical messenger called IL-17A.

Think of IL-17A as a loud, angry siren that screams, "ATTACK! BUILD MORE! FIGHT!"

• In normal skin, the Fibulin 7 safety net acts like a soundproof wall or a muffler. It catches the siren before it reaches the construction workers (stem cells), dampening the noise so they don't panic.
• In the mice without Fibulin 7, the siren screams directly into the workers' ears. The workers hear "BUILD MORE!" and start multiplying uncontrollably, causing the thick, scaly skin seen in psoriasis.

3. The Mechanism: How the Shield Works

The researchers found that Fibulin 7 physically grabs onto the angry siren (IL-17A). It's like a magnet picking up a piece of metal. By holding onto the siren, Fibulin 7 prevents it from docking onto the skin cells' receptors.

• Without Fibulin 7: The siren docks, the cell gets angry, and a chain reaction (involving JNK and p38 pathways) starts. This is like a domino effect where one angry domino knocks over a thousand others, causing chaos.
• With Fibulin 7: The siren is caught in the net. The dominoes stay standing. The cell stays calm.

4. The Human Connection: Real People with Psoriasis

The scientists didn't just stop at mice. They looked at human skin samples from people with psoriasis.

• The Finding: In the angry, red, scaly patches of psoriasis skin, the amount of Fibulin 7 was very low.
• The Correlation: The less Fibulin 7 there was, the more "angry genes" (the ones that cause inflammation) were active.
• The Conclusion: It seems that in many people with psoriasis, their "safety net" has worn thin or disappeared, leaving their skin cells vulnerable to the inflammatory storm.

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Why This Matters (The Takeaway)

Think of Psoriasis not just as a skin rash, but as a city where the construction workers have lost their protective gear and are reacting too strongly to every little alarm.

This paper suggests that Fibulin 7 is that protective gear.

• Current Treatments: Many psoriasis drugs try to turn off the "siren" (IL-17A) from the outside.
• Future Hope: This research suggests we might be able to reinforce the safety net. If we could find a way to boost Fibulin 7 levels in the skin, or create a drug that mimics its "muffling" effect, we might be able to calm the skin down from the inside out, protecting the stem cells and stopping the cycle of inflammation.

In short: Fibulin 7 is the skin's bodyguard. When the bodyguard is missing, the skin cells panic and overreact. Bringing the bodyguard back could be the key to calming the storm.

Technical Summary

1. Problem Statement

Psoriasis is a chronic inflammatory skin disease characterized by epidermal hyperplasia (thickening) and immune cell infiltration. While the role of immune cells and cytokines (particularly IL-17) is well-established, the contribution of the extracellular matrix (ECM) and basement membrane (BM) in protecting tissue stem cells from aberrant inflammation remains elusive. Specifically, it is unclear whether ECM components can act as a barrier to shield epidermal stem cells (EpSCs) from inflammatory cytokines. The authors hypothesize that Fibulin 7 (Fbln7), a matricellular protein known to support EpSC longevity and reside in the BM, may function as a protective factor against inflammatory stress in psoriasis.

2. Methodology

The study employed a multi-faceted approach combining in vivo mouse models, in vitro cell assays, transcriptomics, and human clinical data analysis:

• Animal Model:
  • Subjects: 1-year-old (middle-aged) wild-type (WT) and Fbln7 knockout (KO) mice.
  • Induction: Topical application of Imiquimod (IMQ), a TLR7 agonist, to dorsal skin for 4 days to induce a psoriasis-like phenotype.
  • Lineage Tracing: Utilization of Slc1a3-CreER-tdTomato mice to specifically label and track fast-cycling EpSCs.
• In Vitro Assays:
  • Primary human keratinocytes were stimulated with IL-17A.
  • Recombinant human Fibulin 7 (hF7) was added either pre-mixed with IL-17A, pre-incubated on cells, or added simultaneously to assess signaling modulation.
  • Western blotting was used to measure phosphorylation of p38 MAPK, NF-κB, and JNK.
  • ELISA-based binding assays tested direct interaction between hF7 and various cytokines (IL-17A, OSM, etc.).
• Omics and Bioinformatics:
  • Bulk RNA-seq: Performed on FACS-sorted EpSCs from WT and KO mice under control and IMQ-treated conditions.
  • Human Data Analysis: Re-analysis of public single-cell RNA-seq (scRNA-seq) datasets (GSE173706) and bulk transcriptomic datasets (GSE13355, GSE30999, GSE14905) comparing psoriatic lesions to non-lesional/normal skin.
• Histology and Immunostaining:
  • H&E staining for epidermal thickness.
  • Immunofluorescence for Ki67 (proliferation), p-JNK, and Fibulin 7 localization in mouse and human skin samples.
  • Cytokine arrays and Western blotting on whole skin extracts.

3. Key Results

A. Loss of Fibulin 7 Exacerbates Psoriasis-Like Inflammation

• Phenotype: Fbln7 KO mice treated with IMQ exhibited significantly worse skin phenotypes compared to WT, including increased erythema, scaling, and epidermal thickness (hyperplasia). Dermal thickness remained unchanged.
• Proliferation: KO mice showed a higher density of Ki67+ proliferating cells. Lineage tracing revealed that in KO mice, fast-cycling Slc1a3+ EpSCs underwent asymmetric division, pushing more cells into the suprabasal layers, a hallmark of inflammatory epidermal responses.
• Signaling: KO epidermis displayed significantly elevated phosphorylation of JNK (c-Jun N-terminal kinase) and STAT3 compared to WT after IMQ treatment.

B. Transcriptomic Profiling of EpSCs

• Gene Expression: RNA-seq of EpSCs revealed that while both WT and KO cells responded to IMQ, the loss of Fbln7 amplified the inflammatory response.
• Key Pathways: The 750 genes most strongly upregulated in KO EpSCs were enriched in IL-17 and TNF signaling pathways, as well as MAPK activation.
• Specific Genes: Upregulated genes included inflammatory mediators (Ccl8, Cxcl1/2/3, Ereg, Fos, Junb) and ECM remodeling enzymes (Mmp3/10). Conversely, genes related to fatty acid metabolism and Wnt signaling were downregulated in KO cells.

C. Mechanism: Direct Binding and Sequestration of IL-17A

• Binding Assay: Fibulin 7 was found to directly bind IL-17A and Oncostatin M (OSM) in a dose-dependent manner.
• Signaling Inhibition: In primary human keratinocytes, pre-mixing Fibulin 7 with IL-17A (or adding them simultaneously) significantly suppressed IL-17A-induced activation of p38 MAPK and NF-κB.
• Specificity: The suppression was most potent when Fibulin 7 bound the cytokine before receptor engagement, suggesting a sequestration mechanism rather than a cell-surface receptor blockade. The study notes a stronger suppression of the p38 pathway compared to NF-κB, potentially due to interference with the IL-17RD receptor interaction.
• Chemokine Elevation: Whole-skin analysis showed significantly elevated levels of CXCL5 (a neutrophil chemoattractant) in KO-IMQ skin, suggesting enhanced recruitment of neutrophils in the absence of Fibulin 7.

D. Human Clinical Correlation

• Expression Levels: Analysis of human psoriasis datasets confirmed that FBLN7 mRNA is significantly reduced in lesional skin compared to non-lesional or normal skin.
• Cellular Source: scRNA-seq indicated FBLN7 is primarily expressed in melanocytes and fibroblasts. In lesions, melanocyte expression of FBLN7 was notably decreased.
• Correlation: FBLN7 expression showed a strong inverse correlation with common psoriasis-associated genes (e.g., IL17A, CXCL1, MKI67, STAT3).
• Patient Samples: Immunostaining of human psoriasis patient biopsies confirmed reduced Fibulin 7 at the basement membrane in lesions, accompanied by increased Ki67 and p-JNK.

4. Key Contributions

1. Novel ECM Role: Identifies Fibulin 7 as a critical "shield" in the basement membrane that protects epidermal stem cells from excessive inflammatory signaling during psoriasis.
2. Mechanistic Insight: Demonstrates that Fibulin 7 functions as a cytokine sink, directly binding IL-17A to prevent it from activating keratinocyte receptors (specifically dampening the p38 MAPK pathway).
3. Stem Cell Dynamics: Links the loss of ECM support (Fibulin 7) to the dysregulation of fast-cycling EpSCs, driving them toward asymmetric division and hyperproliferation.
4. Translational Relevance: Validates the findings in human datasets, showing that low FBLN7 is a consistent feature of human psoriatic lesions and correlates with disease severity markers.

5. Significance

This study bridges the gap between ECM biology and immunodermatology. It suggests that the degradation or loss of specific matricellular proteins like Fibulin 7 during inflammation creates a "permissive" environment for cytokine surges (specifically IL-17A), leading to the runaway proliferation characteristic of psoriasis.

• Therapeutic Potential: The findings propose that restoring Fibulin 7 levels or engineering Fibulin 7 variants with enhanced cytokine-sequestration capacity could be a novel therapeutic strategy to dampen IL-17 signaling and protect stem cell niches in inflammatory skin diseases.
• Aging Context: Given that Fibulin 7 levels naturally decline with age and aging skin is more susceptible to inflammation, this mechanism may also explain the increased prevalence of late-onset psoriasis and chronic skin inflammation in the elderly.