Proteomic profiling of whole tissue sections in cardiac ATTR amyloidosis reveals increased extracellular matrix remodeling

Quantitative proteomic analysis of whole cardiac tissue sections in ATTR amyloidosis reveals that, beyond amyloid deposition, the disease is characterized by significant upregulation of extracellular matrix remodeling proteins, complement factors, and coagulation factors, highlighting active tissue restructuring as a key pathological feature with potential therapeutic implications.

The Gist

The Big Picture: A Heart Under Siege

Imagine your heart is a bustling, well-organized city. The buildings are your heart muscle cells (cardiomyocytes), and the streets and parks between them are the Extracellular Matrix (ECM)—a supportive scaffold made of proteins like collagen that keeps everything in shape and allows the heart to stretch and squeeze properly.

In a disease called Cardiac ATTR Amyloidosis, a protein called Transthyretin (TTR) gets "confused." Instead of staying in its proper shape, it folds up incorrectly and turns into hard, sticky bricks called amyloid fibrils. These bricks pile up in the city streets (the heart tissue), crowding out the buildings and making the city stiff and unable to pump blood effectively.

For a long time, doctors thought the disease was just about these bricks piling up passively. But this new study asks: Is the city just sitting there, or is it fighting back?

The Detective Work: Looking at the Whole Neighborhood

Usually, when scientists study this disease, they use a high-powered microscope to cut out only the amyloid bricks (the bricks themselves) to see what they are made of. It's like analyzing a single pile of trash to understand a whole neighborhood.

In this study, the researchers (led by Dr. Vandendriessche and team) took a different approach. They looked at entire tissue sections—the amyloid bricks plus the surrounding streets, buildings, and people. They used a high-tech scanner called Mass Spectrometry (think of it as a super-accurate protein ID machine) to read the "molecular ID cards" of every protein in the tissue.

They compared:

1. The Sick City: Hearts with ATTR amyloidosis.
2. The Healthy City: Normal hearts.
3. The Other Sick City: Hearts with a different type of amyloidosis (AL), just to see if the reaction was unique to ATTR.

The Big Discovery: The City is in "Construction Mode"

The scan revealed something surprising. It wasn't just about the amyloid bricks. The entire neighborhood was in a state of chaotic construction and repair.

The researchers found that the heart tissue was flooding the area with specific tools and workers to try to fix the damage. They identified three main groups of "workers" that were working overtime:

1. The "Demolition and Repair Crew" (ECM Remodeling)

Imagine the amyloid bricks are blocking the streets. The heart tries to clear the way by sending in a demolition crew.

• The Tools: The study found high levels of ADAMTS and MMP proteins. Think of these as chainsaws and jackhammers trying to cut through the old, damaged scaffolding to make room for new repairs.
• The Safety Officers: But you can't just have demolition without safety officers, or the whole city would collapse. The study found high levels of TIMP3, a protein that acts like a safety inspector, telling the chainsaws, "Slow down, don't cut too much!"
• The Result: The heart is constantly trying to rebuild its streets, but the amyloid bricks keep getting in the way. This "tug-of-war" between cutting and rebuilding makes the heart stiff and weak.

2. The "Emergency Response Team" (Complement & Coagulation)

The study also found that the heart's immune system and blood-clotting systems were on high alert.

• The Alarm: Proteins from the complement system (part of the immune system) were turned up loud. It's like the city's fire alarms and police sirens are blaring non-stop because the amyloid bricks are seen as invaders.
• The Traffic Control: Proteins involved in blood clotting were also increased. It's as if the city is trying to put up barricades everywhere to stop the "damage" from spreading, which unfortunately clogs up the roads even more.

3. The "Signature" of the Disease

The study confirmed that this specific mix of "Demolition Crew" + "Safety Officers" + "Emergency Team" is a unique fingerprint of ATTR amyloidosis. When they looked at the other type of amyloidosis (AL), the reaction was similar but much quieter. This means the heart reacts differently depending on which type of amyloid is attacking it.

The Proof: Taking a Closer Look

To make sure their scanner wasn't making mistakes, the researchers went back to the tissue samples and used Immunohistochemistry (a special staining technique) to take photos of the specific proteins.

• They saw TIMP3 (the safety officer) glowing brightly inside the heart muscle cells.
• They saw ADAMTS4 (the demolition tool) glowing strongly right inside the amyloid brick piles.

This confirmed that the heart isn't just a passive victim; it is actively, and perhaps futilely, trying to remodel itself around the amyloid deposits.

Why Does This Matter?

1. New Clues for Treatment: If the heart is stuck in a loop of "demolish and rebuild," maybe we can give it a drug to calm down the repair crew or help the safety officers do their job better. This opens new doors for treating the disease, not just by removing the amyloid, but by helping the heart manage the chaos.
2. Better Diagnosis: The study showed that you don't always need to cut out the tiny amyloid bricks to diagnose the disease. Looking at the whole tissue section (the whole neighborhood) gives enough information to identify the disease and understand what's happening inside the patient.
3. Understanding the "Why": It changes our view of the disease. It's not just "bricks in the wall"; it's a complex battle between the amyloid and the body's attempt to heal itself.

The Bottom Line

This study tells us that in Cardiac ATTR Amyloidosis, the heart is a city under construction. The amyloid deposits are the obstacles, but the real story is the frantic, noisy, and exhausting effort the heart makes to repair the damage. By understanding these "construction workers" (the proteins), scientists hope to find new ways to help the heart rest and recover.

Technical Summary

1. Problem Statement

Cardiac transthyretin amyloidosis (ATTR-CA) is a progressive disease characterized by the deposition of misfolded transthyretin (TTR) fibrils in the myocardium, leading to heart failure with preserved ejection fraction (HFpEF). While the primary pathology is amyloid deposition, the disease involves active biological processes beyond passive accumulation, including extracellular matrix (ECM) remodeling and immune activation.

Current diagnostic gold standards for amyloid typing rely on mass spectrometry (MS) of laser-capture microdissected (LCM) amyloid plaques. However, LCM is labor-intensive and excludes the surrounding tissue microenvironment. There is a need to understand the whole-tissue proteomic landscape of ATTR-CA to uncover pathogenic mechanisms (such as ECM turnover and inflammation) that contribute to myocardial stiffness and dysfunction, and to determine if whole-tissue analysis can provide diagnostic sensitivity comparable to LCM while offering broader biological insights.

2. Methodology

The study employed a quantitative, label-free proteomics approach on formalin-fixed paraffin-embedded (FFPE) whole tissue sections.

• Cohort:
  • ATTR-CA Group: 6 cardiac biopsies (41–78 years old, mostly male).
  • Control Group: 10 unaffected cardiac biopsies (post-transplant or native, histologically normal).
  • Comparator Group: 4 cardiac AL (light chain) amyloidosis biopsies (to assess specificity).
• Sample Preparation:
  • 10 µm thick FFPE sections were collected without microdissection.
  • Proteins were extracted using an SDS-based protocol adapted from the HYPERsol method, involving ultrasonication and heat incubation to reverse formaldehyde crosslinks.
  • Reduction, alkylation, and on-column tryptic digestion were performed using S-trap columns.
• Mass Spectrometry:
  • Platform: Evosep One LC system coupled to a TimsTOF SCP.
  • Acquisition Mode: Data-Independent Acquisition (DIA-PASEF) with a 20 SPD whisper gradient.
  • Data Analysis: Spectra were processed using DIA-NN (v1.8.1) against the human UniProt proteome.
  • Statistics: Differential expression analysis was performed using the limma package in R. Significance was defined as FDR < 0.05 and |log2(fold change)| ≥ 2. Missing values were imputed using the DEP package.
• Validation:
  • Immunohistochemistry (IHC): Performed on FFPE sections from an ATTR case and a control to validate the expression and localization of ADAMTS4 and TIMP3.
• Bioinformatics: Gene Ontology (GO) enrichment analysis was conducted using DAVID.

3. Key Contributions

• Whole-Tissue Proteomics: Demonstrated that quantitative proteomics on whole FFPE sections (without LCM) is a viable strategy for amyloid typing and provides a comprehensive view of the amyloid-associated proteome, including the surrounding stroma and cardiomyocytes.
• Discovery of ECM Remodeling Signatures: Identified a robust, reproducible signature of ECM remodeling in ATTR-CA, specifically highlighting the upregulation of metalloproteinases (ADAMTS, MMPs) and their inhibitors (TIMP3), distinct from simple amyloid deposition.
• Specificity Analysis: Compared ATTR-CA against AL-CA and controls, revealing that while complement and coagulation activation occurs in both, the magnitude of ECM remodeling (specifically ADAMTS and TIMP3 upregulation) is more pronounced and specific to ATTR-CA.
• Diagnostic Utility: Showed that whole-tissue proteomics can correctly classify ATTR cases, suggesting a potential alternative to LCM in settings where tissue is limited or microdissection is not feasible.

4. Key Results

• Proteome Coverage: Identified 4,291 proteins; 3,824 were reliably quantified.
• Differential Expression:
  • Upregulated: 519 proteins in ATTR-CA vs. controls.
  • Downregulated: 75 proteins.
• Amyloid Signature: TTR was the most abundant amyloidogenic protein. Signature proteins (ApoE, ApoAIV, SAP, Vitronectin, Clusterin) were significantly elevated, confirming the diagnosis.
• Pathway Enrichment (GO):
  • Immune/Complement: Significant upregulation of the complement system (C1q, C2, C4, C6–C9, CFD, CFH) and coagulation factors (F8, F10, F11, F12, KLKB1). Thrombomodulin was downregulated.
  • ECM Remodeling: Significant upregulation of ADAMTS1, ADAMTS4, ADAMTS5, and ADAMTSL3. MMP19 and MMP23B were also upregulated.
  • Collagen: Increased levels of multiple collagen types (COL3A1, COL5A1, COL6A1, etc.) and procollagen processing enzymes.
• Validation (IHC):
  • TIMP3: Showed strong cytoplasmic staining in cardiomyocytes and weaker expression in amyloid deposits in ATTR tissue, contrasting with patchy/weak staining in controls.
  • ADAMTS4: Showed strong, homogeneous staining specifically within amyloid deposits in ATTR tissue, with weak staining in controls.
• Heterogeneity: A subset of ATTR samples (3/6) showed additional upregulation in metabolic and stress pathways (PPAR signaling, lysosome activity), though this did not correlate with clinical parameters like age or amyloid load.
• Comparison with AL-CA: While AL-CA showed some complement/coagulation upregulation, the ECM remodeling signature (ADAMTS/TIMP3) was significantly more pronounced in ATTR-CA.

5. Significance and Implications

• Pathophysiological Insight: The study shifts the view of ATTR-CA from a passive deposition disease to an active process involving protease-inhibitor dynamics. The upregulation of ADAMTSs and TIMP3 suggests a local injury response and active matrix turnover, which likely contributes to myocardial stiffness and dysfunction beyond the physical presence of fibrils.
• Therapeutic Targets: The identified proteins (ADAMTS4, TIMP3, specific collagens) represent potential novel therapeutic targets to modulate ECM remodeling and improve cardiac function.
• Biomarker Potential: The distinct proteomic signature of ECM remodeling could serve as a basis for developing biomarkers to monitor disease progression or response to therapy.
• Diagnostic Workflow: The findings support the use of whole-tissue proteomics as a robust diagnostic tool that captures the broader disease context, potentially reducing the reliance on labor-intensive LCM in clinical settings, provided that the background proteome is well-understood.

Limitations: The study had a small sample size (n=6 ATTR), and controls included immunosuppressed transplant patients, which may influence complement levels. Future studies require larger cohorts and non-amyloid fibrosis controls to fully validate the specificity of these findings.