sPRR signaling in macrophages via the AT1R/Yap/Taz axis to induce renal fibrosis

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This is an AI-generated explanation of a preprint that has not been peer-reviewed.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

The Big Picture: A Construction Site Gone Wrong

Imagine your kidney is a busy construction site. Its job is to filter waste and keep your body running smoothly.

Sometimes, a blockage happens (like a pipe getting clogged). This is called Obstructive Ureteral Obstruction (UUO). When this happens, the construction site gets chaotic. Instead of just fixing the pipe, the body sends in a massive crew of "repair workers" (immune cells called macrophages).

Usually, these workers are helpful. But in this specific scenario, they get confused. They switch from "fixing the pipe" mode to "over-building" mode. They start laying down too much concrete and steel (scar tissue), which eventually ruins the building. This is Kidney Fibrosis—essentially, the kidney turning into a brick wall and stopping it from working.

This study asks: What is the signal that tells these repair workers to stop fixing and start over-building?

The Culprit: The "PRR" Foreman

The researchers found a specific protein called PRR (Prorenin Receptor). Think of PRR as a foreman standing on the roof of the kidney's "collecting duct" (the main drainage pipe).

The Signal: When the kidney is injured, this foreman gets activated. He chops off a piece of himself and sends it out as a messenger called sPRR (soluble PRR).
The Delivery: This messenger (sPRR) floats through the blood and lands on the repair workers (macrophages) on the ground.
The Order: The messenger tells the workers: "Don't just clean up! Build a fortress!" This causes the workers to turn into M2 Macrophages—the "scar-building" type.

The Chain Reaction: The "Yap/Taz" Switch

Once the messenger (sPRR) hits the worker, it flips a switch inside the worker's brain. This switch is a pair of proteins called Yap and Taz.

Without the switch: The worker stays calm and just cleans up.
With the switch flipped: The worker goes into overdrive, pumping out chemicals that build scar tissue.

The study discovered that the messenger (sPRR) flips this switch by talking to another protein on the worker's face called AT1R. It's like a key (sPRR) fitting into a specific lock (AT1R) to turn on the engine (Yap/Taz).

The Experiment: Turning Off the Foreman

To prove this theory, the scientists did two main things:

1. Removing the Foreman (Genetic Knockout):
They took a group of mice and genetically deleted the "foreman" (PRR) from the kidney's roof.

Result: When these mice had a blocked kidney, the messenger (sPRR) never got sent out. The repair workers (macrophages) didn't get the "build a fortress" order. They stayed calm, built very little scar tissue, and the kidney survived much better.

2. Jamming the Lock (Drug Treatment):
They took normal mice and gave them a drug (PF-429242) that stops the foreman from chopping off the messenger.

Result: Even though the mice had a blocked kidney, no messenger was sent. The workers didn't get the order to build scars. The fibrosis was significantly reduced.

3. The Lab Test (In Vitro):
They took immune cells out of the body and put them in a dish.

They added the messenger (sPRR) directly. The cells immediately turned into scar-builders.
They added a blocker (Losartan) that jams the lock (AT1R). The messenger couldn't get in, and the cells stayed calm.

The Conclusion: A New Way to Fix the Building

The study concludes that the PRR foreman in the kidney is a major villain in kidney scarring. He sends a signal (sPRR) that hijacks the repair workers, flips their internal switch (Yap/Taz), and forces them to build damaging scars.

Why does this matter?
Currently, there is no cure for kidney scarring (fibrosis). Once the kidney turns to scar tissue, it's gone forever. This study suggests a new way to treat Chronic Kidney Disease:

Stop the Foreman: Prevent the PRR from sending the messenger.
Jam the Lock: Block the AT1R receptor so the messenger can't get in.
Flip the Switch: Turn off the Yap/Taz engine inside the cells.

If we can do any of these things, we might be able to stop the kidney from turning into a brick wall, keeping it functional for much longer.

Summary Analogy

The Kidney: A house with a clogged drain.
The Macrophages: The repair crew.
PRR/sPRR: The foreman shouting "Build a wall!"
AT1R: The megaphone the foreman uses.
Yap/Taz: The workers' internal alarm system that makes them start building.
The Study: Proving that if you silence the foreman or jam the megaphone, the workers stop building walls, and the house (kidney) stays safe.

1. Problem Statement

Chronic Kidney Disease (CKD) inevitably progresses to kidney fibrosis, the final common pathway leading to end-stage renal disease. While the role of myeloid cells (specifically macrophages) in fibrosis is recognized, the precise molecular mechanisms driving their transition to a pro-fibrotic, alternatively activated (M2) state remain elusive.

Context: The (Pro)renin receptor (PRR) is abundant in the kidney's collecting duct (CD). Previous work by the authors established that CD-specific PRR activation mediates obstructive renal fibrosis in Unilateral Ureteral Obstruction (UUO) models.
Gap: The specific signaling pathway by which CD-derived PRR influences macrophage polarization and subsequent fibrosis was unknown. Specifically, the role of soluble PRR (sPRR), the Hippo pathway effectors Yap/Taz, and the Angiotensin II Type 1 Receptor (AT1R) in this context had not been elucidated.

2. Methodology

The study employed a combination of in vivo genetic models, pharmacological interventions, and in vitro cell culture experiments.

Animal Models:
- Genetic Knockout: Male mice with Collecting Duct-specific PRR deletion (CD PRR KO) and floxed controls were subjected to UUO surgery.
- Pharmacological Inhibition: Wild-type C57BL/6 mice underwent UUO and were treated with an osmotic minipump delivering PF-429242 (a Site-1 Protease [S1P] inhibitor) to block the cleavage of full-length PRR into sPRR, or a vehicle control.
- Timeline: Tissues and blood were harvested 7 days post-surgery.
In Vitro Models:
- Cell Type: Bone Marrow-Derived Macrophages (BMDMs).
- Stimulation: BMDMs were treated with recombinant sPRR-His (10 nM).
- Interventions: Cells were pre-treated with specific inhibitors to block key pathway components:
  - Losartan: To block AT1R.
  - Verteporfin: To inhibit Yap/Taz activity.
  - PF-429242: To inhibit S1P (used in combination with TGFβ1 in some assays).
Analytical Techniques:
- Immunofluorescence & Histology: Staining for macrophage markers (F4/80), M2 markers (Mannose Receptor/MR), and Yap/Taz localization. Masson's trichrome and Sirius Red staining for fibrosis quantification.
- Western Blot: Analysis of protein expression for PRR, MR, Yap/Taz, Fibronectin (FN), $\alpha$ -SMA, and phosphorylated Yap.
- qRT-PCR: Quantification of mRNA levels for M2 markers (Arg-1, MR, Fizz-1, YM1), Yap/Taz target genes (Ctgf, Ankrd1), and fibrotic cytokines.
- ELISA: Measurement of plasma sPRR concentrations.
- Cell Sorting: Macrophages were isolated from spleens and obstructed kidneys using CD115 microbeads for specific analysis.

3. Key Contributions

Mechanistic Elucidation: The study defines a novel signaling axis: CD PRR $\rightarrow$ S1P-derived sPRR $\rightarrow$ AT1R $\rightarrow$ Yap/Taz $\rightarrow$ Macrophage M2 Polarization $\rightarrow$ Renal Fibrosis.
Macrophage Specificity: It demonstrates that sPRR acts directly on macrophages (via AT1R) to induce an M2 phenotype, rather than acting solely on tubular cells.
Therapeutic Target Validation: It identifies the S1P cleavage step and the downstream Yap/Taz axis as potential therapeutic targets to halt fibrosis progression.

4. Key Results

A. Genetic Deletion of CD PRR Attenuates Fibrosis and M2 Polarization

Fibrosis: CD PRR KO mice subjected to UUO showed significantly reduced renal fibrosis compared to floxed controls.
Macrophage Accumulation: F4/80 staining revealed markedly reduced macrophage infiltration in the obstructed kidneys of CD PRR KO mice.
M2 Polarization: Macrophages from CD PRR KO mice exhibited significantly lower expression of M2 markers (MR, Arg-1, Fizz-1, YM1) at both protein and mRNA levels.
Yap/Taz Axis: The expression and nuclear translocation of Yap/Taz were significantly suppressed in macrophages from CD PRR KO mice, along with reduced expression of their target genes (Ctgf, Ankrd1).
Plasma sPRR: CD PRR KO mice had significantly lower plasma sPRR levels post-UUO.

B. Pharmacological Inhibition of S1P (PF-429242) Mimics Genetic Knockout

Treatment with PF-429242 in wild-type UUO mice reduced plasma sPRR levels.
This treatment resulted in decreased macrophage accumulation, reduced M2 marker expression, and suppressed Yap/Taz activation in obstructed kidneys, confirming that S1P-derived sPRR is the critical mediator.

C. Direct Effect of sPRR on Macrophages (In Vitro)

Induction of M2: Treatment of BMDMs with sPRR-His directly induced M2 polarization (upregulation of MR, Arg-1, etc.) and fibrotic markers (FN, $\alpha$ -SMA).
AT1R Dependence: The pro-fibrotic and M2-inducing effects of sPRR-His were abolished by Losartan (AT1R blocker). This confirms sPRR signals through AT1R on macrophages.
Yap/Taz Dependence: sPRR-His treatment increased Yap/Taz expression and nuclear translocation. This effect was blocked by Verteporfin (Yap inhibitor), which subsequently prevented sPRR-induced M2 polarization and fibrotic marker expression.

D. The Signaling Cascade

The study establishes the following sequence:

Obstruction induces CD PRR expression.
S1P cleaves PRR to release sPRR.
sPRR binds to AT1R on macrophages.
AT1R activation triggers the Yap/Taz pathway.
Activated Yap/Taz drives M2 polarization and the secretion of pro-fibrotic factors, exacerbating renal fibrosis.

5. Significance and Clinical Implications

Novel Pathophysiology: This study shifts the paradigm of renal fibrosis by highlighting a non-canonical role of the collecting duct in regulating immune cell polarization via the renin-angiotensin system components (PRR/sPRR).
Therapeutic Potential: The findings suggest that targeting the sPRR generation (S1P inhibition) or the downstream Yap/Taz axis could provide new therapeutic strategies to reverse or halt the progression of CKD to end-stage renal disease.
Biomarker Relevance: The correlation between plasma sPRR levels and fibrosis severity supports the potential utility of sPRR as a biomarker for disease progression.

In conclusion, the paper provides robust evidence that CD-derived sPRR acts as a critical link between renal tubular injury and macrophage-driven fibrosis, operating through an AT1R/Yap/Taz signaling axis.