Integrated Single-Fiber Multi-Omics Links an Inflammatory-Associated Myofiber State to Altered Myosin Dynamics in Patients with ICU-acquired weakness

This study utilizes integrated single-fiber multi-omics and functional profiling to identify a conserved inflammatory myofiber state in ICU-acquired weakness that links transcriptional and proteomic remodeling to disrupted myosin dynamics and muscle dysfunction.

The Gist

Imagine your body's muscles are like a massive, bustling city made of millions of individual "factories" (your muscle fibers). In a healthy person, these factories run smoothly, producing the energy and movement needed to walk, stand, and live.

But when someone gets very sick and ends up in the Intensive Care Unit (ICU), something strange happens. Even if they survive the illness, their muscles often become weak and wasted away. This is called ICU-acquired weakness. Doctors have known this happens for a long time, but they've struggled to understand exactly why it happens inside the tiny muscle cells. Is it a lack of protein? A broken engine? A chemical imbalance?

This paper is like a high-tech detective story that finally solved the mystery by looking at the crime scene with a microscope so powerful it could see the molecular level.

The Detective's New Tool: The "One-Fiber" Investigation

Usually, when scientists study muscle, they take a big chunk of tissue, grind it all up, and look at the average. It's like taking a smoothie of a whole city and trying to guess what the bakery, the power plant, and the school look like individually. You lose the details.

The researchers in this study invented a clever new method. They took single muscle fibers (one tiny factory at a time) from ICU patients and healthy people. They didn't just look at one thing; they did a "triple-threat" investigation on the same fiber:

1. The Blueprint (RNA): What instructions are the cell reading?
2. The Machinery (Proteins): What actual tools and parts are being built?
3. The Engine Performance (Function): How fast is the motor turning over?

The Discovery: The "Stressed-Out" Fiber

When they looked at the data, they found something fascinating. Not every muscle fiber in an ICU patient was broken. But, there was a specific group of fibers that had gone into a very distinct "survival mode."

Think of these fibers as factories that have been invaded by a storm.

• The Alarm Bells: The "blueprints" (RNA) showed the factory was screaming about inflammation and stress. It was sounding alarms for immune cells and preparing for a fight.
• The Shift in Priorities: The "machinery" (proteins) told a different story. The factory stopped building the fancy exterior decorations (membrane signaling) and instead focused entirely on internal repairs. They were building more mitochondria (the power plants) and more assembly lines (ribosomes) to fix themselves.
• The Result: The factory was trying to save energy and repair damage, but in doing so, it changed how its main engine worked.

The Broken Engine: The "Super-Relaxed" State

Here is the most important part. Muscle fibers have a main engine called myosin. This engine has two modes:

1. Active Mode: Ready to move and pull (like a car in gear).
2. Super-Relaxed Mode: A "sleeping" mode where the engine idles very slowly to save fuel (like a car in park with the engine off).

In healthy muscles, the engine switches between these modes perfectly. But in these "stressed-out" ICU fibers, the engine got stuck in the "Super-Relaxed" mode.

Imagine a car that is stuck in park. It's very fuel-efficient (it saves energy), but it won't move. The researchers found that the "sleeping" engine in these sick fibers took much longer to wake up and start working. This explains why the patients are weak: their muscles aren't necessarily broken or missing parts; they are just too cautious to move. They are hoarding energy to survive the crisis, sacrificing the ability to generate power.

The Big Picture: Why This Matters

This study is a breakthrough because it connected the dots between three different worlds:

1. The Stress Signal: The cell knows it's sick (Inflammation).
2. The Molecular Change: The cell changes its parts to survive (Proteins).
3. The Physical Result: The muscle gets weak because the engine won't wake up (Function).

The Analogy of the City:
Imagine a city under attack. The mayor (the cell) decides to shut down all the parks and restaurants (membrane signaling) and redirect all the workers to the power plant and the repair shop (mitochondria and ribosomes). The city becomes incredibly efficient at surviving, but it can't throw a parade or move heavy trucks (contractile force) because everyone is too busy fixing the walls.

What's Next?

This research gives doctors a new target. Instead of just trying to feed the muscles or stop the inflammation generally, they might be able to develop drugs that specifically "wake up" that stuck engine, helping the muscle fibers switch from "survival mode" back to "movement mode."

In short: ICU weakness isn't just about muscles wasting away; it's about muscles getting stuck in a deep, energy-saving sleep. This study found the alarm clock that might wake them up.

Technical Summary

1. Problem Statement

ICU-Acquired Weakness (ICU-AW) is a pervasive complication in critically ill patients, characterized by rapid skeletal muscle atrophy and dysfunction that significantly worsens survival and recovery. While previous studies have identified broad molecular changes (e.g., inflammation, proteolysis, mitochondrial dysfunction) in bulk muscle tissue, these approaches fail to capture cellular heterogeneity.

• The Gap: Skeletal muscle is a complex tissue with diverse fiber types and states. Bulk analyses obscure specific cell states, and existing single-cell/single-fiber studies typically analyze only one molecular layer (transcriptomics or proteomics or function) per fiber.
• The Challenge: There is a lack of direct evidence linking specific molecular states (RNA/protein) to functional contractile defects (myosin kinetics) within the same individual human myofiber in the context of critical illness.

2. Methodology

The authors developed and applied a novel integrated single-fiber multi-omics workflow to bridge molecular profiling with functional biophysics.

• Cohort: 16 human subjects (8 ICU-AW patients, 8 orthopedic controls). Muscle biopsies were taken from the quadriceps femoris.
• Single-Fiber Isolation: Individual myofibers were manually isolated and split into two segments:
  1. Segment A (Functional): Subjected to Mant-ATP chase experiments to measure myosin ATP turnover kinetics (Super-Relaxed [SRX] and Disordered-Relaxed [DRX] states).
  2. Segment B (Multi-Omics): Processed for simultaneous transcriptomics (RNA-seq) and proteomics (LC-MS/MS).
• Data Integration: Spatial registration ensured that the functional data from one segment was linked to the omic data from the other segment of the same fiber.
• Bioinformatics:
  • Transcriptomics: Pseudobulk analysis by donor to account for nested structure; Hierarchical clustering to identify fiber subpopulations.
  • Proteomics: DIA-PASEF mass spectrometry with imputation and normalization.
  • Integration: Intersection of Differentially Expressed Genes (DEGs) and Proteins (DEPs) to find concordant features.
  • Scoring: A composite RNA-protein score was calculated for each fiber based on 19 concordantly altered features and correlated with functional kinetic parameters.

3. Key Contributions

• Novel Workflow: First study to sequentially perform functional myosin kinetics, transcriptomics, and proteomics on the same single human myofiber.
• Discovery of a Conserved State: Identification of a specific, disease-enriched "Inflammatory" myofiber state within the heterogeneous ICU-AW population that is not defined by fiber type (Myosin Heavy Chain isoform).
• Multi-Omic Concordance: Successfully linked transcriptional inflammatory signatures to proteomic shifts and functional energetic defects, overcoming the "noise" of bulk tissue analysis.

4. Key Results

A. Transcriptomic Landscape

• No Global Shifts: Direct donor-level comparison showed no genome-wide differentially expressed genes (DEGs).
• The "Inflammatory" Cluster: Hierarchical clustering revealed a distinct subcluster of fibers (32 ICU-AW fibers vs. 1 control) characterized by:
  • Upregulation: Inflammatory pathways (ROS, IL6-JAK-STAT3, cytokine signaling), immune activation, and chemotaxis.
  • Downregulation: Metabolic homeostasis, myogenesis, and developmental pathways.
• Specificity: This state was present across all ICU-AW donors but not uniformly in all fibers, indicating a specific stress response rather than a global tissue shift.

B. Proteomic Landscape

• Shift in Priorities: The "Inflammatory" cluster showed a proteomic shift toward intracellular maintenance and away from membrane signaling.
  • Upregulated: Mitochondrial machinery, ribosomal biogenesis, translational machinery, and wound repair.
  • Downregulated: Membrane-associated processes and surface proteins.
• Post-Transcriptional Regulation: Many transcriptomic changes did not match protein levels, suggesting active post-transcriptional control (e.g., selective protein stabilization or degradation).

C. RNA-Protein Integration (The 19 Features)

The study identified 19 features significantly altered at both RNA and protein levels, linking inflammation to structural remodeling:

• Concordantly Upregulated: ACTN4, MYL9, VTN, S100A11 (cytoskeletal anchoring/dynamics), RPS9 (translation), and immune effectors (CTSG, TPSB2).
• Concordantly Downregulated: IDI1 (lipid metabolism/membrane biogenesis).
• Discordant Regulation: Some genes showed decreased mRNA but increased protein (e.g., AUH, NCK2), indicating selective protein stabilization, while others showed increased mRNA but decreased protein, suggesting enhanced degradation.

D. Functional Correlation (Myosin Dynamics)

• SRX State Alteration: Fibers belonging to the "Inflammatory" cluster displayed a significantly prolonged ATP turnover time in the Super-Relaxed (SRX) state (T2 parameter).
• Interpretation: A prolonged SRX time constant indicates reduced basal ATPase activity. While this saves energy, it suggests a fundamental alteration in the myosin motor's ability to transition from a resting to an active state.
• Correlation: There was a strong correlation between the composite multi-omic score (based on the 19 features) and the SRX time constant, directly linking the molecular phenotype to bioenergetic dysfunction.

5. Significance and Implications

• Mechanistic Insight: The study defines a discrete molecular program in ICU-AW where the muscle fiber prioritizes bioenergetic survival (mitochondrial maintenance, reduced basal ATP consumption) at the expense of contractile readiness. The shift toward the SRX state is a compensatory mechanism to energy scarcity but contributes to functional weakness.
• Clinical Relevance:
  • Identifies a specific "Inflammatory" fiber state that could serve as a biomarker for patient stratification.
  • Suggests that therapeutic interventions should target mitochondrial dysfunction and inflammatory stress responses to restore myosin dynamics, rather than just treating atrophy.
• Methodological Advance: Demonstrates that single-fiber multi-omics is essential for resolving cellular heterogeneity in complex diseases. It validates that molecular signals can be directly interpreted within their native biomechanical context, a capability missing in traditional bulk or single-layer single-cell studies.

In conclusion, this paper establishes that ICU-AW is not merely a uniform loss of muscle mass but involves a specific, conserved myofiber state characterized by inflammatory signaling, intracellular remodeling, and altered myosin energetics, providing a new framework for understanding and treating critical illness myopathy.