🧬 biology

Mechanosensing at the endoplasmic reticulum by IRE1

This study identifies the endoplasmic reticulum as an autonomous mechanosensitive organelle where the transmembrane protein IRE1 directly senses membrane tension to trigger JNK signaling and enhance global protein synthesis, thereby linking mechanical forces to muscle adaptation independently of its canonical unfolded protein response role.

Original authors: Hesso Farhan, Michaela Mayr, Luiz Garcia-Souza, Murphy McDermott, Utku Horzum, Yannick Frey, Margot Haun, Stephan Geley, Chao Jiang, Mattias Luber, Anush Bakunts, Camilla Pirolo, Alesia Yakimchyk, Kla

Published 2026-07-01

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CC BY 4.0

Original authors: Hesso Farhan, Michaela Mayr, Luiz Garcia-Souza, Murphy McDermott, Utku Horzum, Yannick Frey, Margot Haun, Stephan Geley, Chao Jiang, Mattias Luber, Anush Bakunts, Camilla Pirolo, Alesia Yakimchyk, Klaus Liedl, Wylie Stroberg, Timo Betz, Liam Holt

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

Imagine your body is a bustling city. For a long time, scientists believed that the city's "sensors" for physical pressure and movement were only located at the city limits (the plasma membrane) and in the central command center (the nucleus). They thought these were the only places that could feel a bump, a stretch, or a squeeze and send a message to the rest of the city to react.

This paper introduces a new, surprising discovery: The city's massive warehouse district (the Endoplasmic Reticulum, or ER) has its own independent sensors, too. Specifically, the researchers found that a protein called IRE1, which usually acts as a "quality control inspector" for misfolded proteins, has a secret second job: it is a mechanical pressure gauge.

Here is how the study breaks down, using simple analogies:

1. The Discovery: The Warehouse Feels the Squeeze

The researchers squeezed cells (like putting a soft sponge in a tight box) to see what happened. They found that when the cell's internal "warehouse" (the ER) got squished, the IRE1 protein didn't just sit there. It immediately sensed the pressure.

The Analogy: Think of the ER membrane as a rubber balloon. When you squeeze the balloon, the rubber stretches tight. IRE1 is like a tiny sensor glued to that rubber. When the rubber stretches (increases in tension), IRE1 snaps into action.

2. How It Works: The "Stretch" Switch

Usually, IRE1 only wakes up when it sees "broken" or "misfolded" proteins (like a factory inspector finding defective products). But this study shows that IRE1 wakes up even when the products are perfect, simply because the "floor" it stands on (the membrane) is being stretched.

The Mechanism: The researchers found that IRE1 has a specific part (a tryptophan molecule, labeled W457) that acts like a deep-seated anchor in the rubber. When the rubber stretches, this anchor shifts, causing IRE1 molecules to clump together (dimerize) and turn on.
The Speed: Unlike some sensors that react in a split second (like a reflex), this one takes a few minutes to "clump up" and send a signal. It's more like a slow-acting alarm that builds up as the pressure continues.

3. What Happens Next: The "Build More" Signal

Once IRE1 feels the stretch, it doesn't call for a shutdown or a repair crew (which is what it usually does with bad proteins). Instead, it sends a different message: "Build more!"

The Pathway: It activates a messenger called JNK.
The Result: This messenger tells the cell to ramp up protein production. The cell starts manufacturing more proteins to handle the new physical demands.
Crucial Point: This happens without the usual "quality control" steps. The cell isn't fixing broken things; it's proactively making more stuff because it feels the physical stress.

4. The Muscle Connection: Training Makes You Stronger

To prove this isn't just a lab trick, the researchers looked at muscle tissue.

The Experiment: They took engineered muscle tissue and either gave it electrical pulses (to make it contract) or gently stretched it.
The Finding: Just like the squeezed cells, the muscle tissue's IRE1 sensors turned on. This triggered the "build more" signal, leading to increased protein synthesis.
The Proof: When they used a drug to block IRE1, the muscle tissue could no longer get stronger after training. The "pressure sensor" was broken, so the muscle didn't know it needed to build more strength.

5. What It Is NOT

The paper is very careful to say what this is not:

It is not caused by broken DNA.
It is not caused by the nucleus sensing the pressure and telling the ER what to do.
It is not the usual "stress response" where the cell tries to fix misfolded proteins.
It is not dependent on ion channels (like Piezo) that usually sense touch.

The Big Picture

This paper rewrites the rulebook on how cells feel physical force. It shows that the Endoplasmic Reticulum is an independent "feeler" that can sense when the cell is being squeezed or stretched. When it feels this tension, it uses the IRE1 protein to flip a switch that tells the cell to produce more proteins, helping the cell (and muscles) adapt and get stronger in response to physical training or stress.

In short: Your cells have a built-in "gym coach" inside their warehouse that senses when you are working out and tells them to build more muscle, all without needing to check for broken parts first.

Technical Summary: Mechanosensing at the Endoplasmic Reticulum by IRE1

Problem and Hypothesis
While the plasma membrane and nucleus are well-established sites for mechanotransduction, the role of intracellular organelles in sensing mechanical forces remains largely unexplored. The endoplasmic reticulum (ER), the largest organelle in eukaryotic cells, is known to undergo morphological changes and functional alterations under mechanical stress, yet the molecular mechanisms enabling ER-based mechanosensing are elusive. The authors hypothesized that IRE1, a transmembrane kinase traditionally known as a sensor of protein misfolding (the Unfolded Protein Response, or UPR), might also function as a mechanosensor capable of detecting mechanical perturbations of the ER membrane.

Methodology
The study employed a multidisciplinary approach combining cell biology, biophysics, and computational modeling:

Mechanical Perturbation: Cells (HeLa, HEK293A, fibroblasts, and various cancer lines) were subjected to acute mechanical stress via physical confinement (3 µm height), compression (0.23 kPa), and uniaxial stretching.
Direct ER Stimulation: To isolate ER mechanics from plasma membrane signaling, the authors utilized hypoosmotic shock to induce ER swelling and micromanipulation of isolated Giant ER Vesicles (GERVs) via micropipette aspiration.
Readouts: IRE1 activation was assessed via immunoblotting for phosphorylation (pIRE1), live-cell imaging of IRE1-GFP clustering, and functional assays for downstream signaling (JNK activation, XBP1 splicing, RIDD activity).
Membrane Tension Sensing: A genetically encoded tension sensor (MscL-cpGFP) localized to the ER was used to measure lateral membrane tension changes. Fluorescence Recovery After Photobleaching (FRAP) of ER transmembrane proteins (Sec61b, Calnexin) was used to assess membrane fluidity.
Translation and Signaling: Global protein synthesis was measured using the SUnSET assay (puromycin incorporation). The involvement of specific pathways (UPR, ion channels, cytoskeleton, nuclear mechanics) was tested using pharmacological inhibitors (KIRA6, SP600125, Gd3+, etc.) and siRNA-mediated knockdowns.
Muscle Models: Engineered 3D skeletal muscle tissues were subjected to electrical stimulation and passive stretch to test IRE1 function in a physiologically relevant context.
Computational Modeling: Reaction-diffusion models and molecular dynamics (MD) simulations (using AlphaFold 3 and Amber24) were employed to understand IRE1 oligomerization kinetics and the structural role of the transmembrane residue W457.

Key Results

IRE1 is a Mechanosensor: Acute mechanical confinement, compression, and stretching induced rapid phosphorylation of IRE1 and the formation of IRE1 clusters within minutes. This response was reversible upon release of stress and observed across multiple cell types.
Independence from Canonical UPR: Mechano-activation of IRE1 occurred without the accumulation of misfolded proteins (no TPE-MI signal), without PERK or ATF6 activation, and without XBP1 splicing or RIDD activity. The response was independent of known mechanosensitive pathways (Piezo channels, ERK, Akt, c-Abl) and nuclear mechanosensing (cPLA2, lamin A).
Direct Sensing of Membrane Tension:
- Hypoosmotic shock and micropipette aspiration of GERVs directly induced IRE1 clustering, confirming that ER membrane tension alone is sufficient for activation.
- The MscL-cpGFP sensor confirmed a rapid increase in lateral membrane tension upon confinement.
- FRAP experiments showed accelerated diffusion of ER proteins under confinement, consistent with membrane thinning/stretching.
- Reducing membrane tension via oleic acid treatment abolished mechano-activation of IRE1, while the response to proteotoxic stress (thapsigargin) remained intact.
Structural Mechanism: The transmembrane residue W457 was identified as critical for mechano-sensing. The W457A mutant failed to cluster or activate under mechanical stress but retained responsiveness to proteotoxic stress. MD simulations suggested W457 acts as an interaction hub within the transmembrane helix, potentially sensing lipid packing density rather than forming direct dimer interfaces via $\pi$ - $\pi$ stacking.
Cytoskeletal Transmission: Intermediate filaments (vimentin) were identified as the primary cytoskeletal component transmitting force from the cell surface to the ER; depletion of vimentin, but not actin or microtubules, abolished IRE1 mechanosensing.
Downstream Signaling and Translation: Mechano-activated IRE1 triggered JNK signaling, which in turn drove a global increase in protein translation. This process was independent of XBP1 splicing and the RNase activity of IRE1. The increase in translation was blocked by JNK inhibition or IRE1 depletion.
Muscle Adaptation: In engineered skeletal muscle tissues, both electrical stimulation and passive stretch activated IRE1 and JNK, leading to increased protein synthesis. Crucially, inhibiting IRE1 prevented the training-induced increase in contractile force, linking IRE1-mediated mechanosensing to muscle hypertrophy and functional adaptation.

Significance and Claims
The paper claims to identify the ER as an autonomous mechanosensitive organelle and IRE1 as a specific ER-resident mechanosensor. The authors propose a non-canonical role for IRE1 that is distinct from its function in the UPR: sensing lateral membrane tension to regulate protein synthesis via a JNK-dependent axis. This mechanism allows cells to couple mechanical forces directly to anabolic output, facilitating adaptation in mechanically stressed tissues such as skeletal muscle. The study expands the paradigm of mechanosensing beyond the plasma membrane and nucleus, suggesting that organelle-specific mechanotransduction is a fundamental component of cellular homeostasis. The authors note limitations, including the focus on short-term stress and the use of overexpressed proteins for imaging, but emphasize the physiological relevance demonstrated in muscle tissue models.