1. Binding: Relaxin binds to the RXFP1 receptor on the surface of endothelial cells. This activates the G protein Giα2, which is coupled to the receptor.

2. Downstream Signaling: Giα2 inhibits adenylyl cyclase, an enzyme that normally produces cyclic AMP (cAMP). Reduced cAMP levels activate another enzyme, phosphoinositide 3-kinase (PI3K), which triggers a cascade of signaling events:

• Akt Activation: PI3K activates Akt, a protein that promotes cell survival and growth.
• eNOS Phosphorylation: Akt phosphorylates endothelial nitric oxide synthase (eNOS), the enzyme responsible for NO production.

3. NO Production and Vasodilation: Phosphorylated eNOS becomes more efficient at converting L-arginine into nitric oxide (NO). NO diffuses to nearby smooth muscle cells in the blood vessel wall, causing them to relax and dilate the blood vessel.

1. RXFP1 Receptor and G Protein Signaling:

• Binding: Relaxin binds to the RXFP1 receptor expressed on renal fibroblasts. This activates the G protein Giα2, which inhibits adenylyl cyclase, reducing intracellular cAMP levels.

2. Downstream Pathways:

a) Phosphoinositide 3-kinase (PI3K)/Akt pathway:

• Inhibition of Proliferation: Reduced cAMP activates PI3K/Akt, a pro-survival pathway. Akt inhibits cyclin-dependent kinases (CDKs), cell cycle regulatory proteins, blocking fibroblast proliferation.
• MMP-9 and -13 Upregulation: Akt also stimulates the transcription of MMP-9 and -13 genes, promoting their production and release. MMPs are enzymes that degrade extracellular matrix (ECM) components, contributing to tissue remodeling.

b) Mitogen-activated protein kinase (MAPK) pathways:

• ERK1/2 and p38 MAPK: Relaxin can activate these pathways depending on the cell type and context. ERK1/2 may contribute to MMP-9 expression, while p38 MAPK can have context-dependent effects, inhibiting proliferation in some models.

c) Nitric oxide (NO)/cyclic GMP (cGMP) pathway:

• Anti-fibrotic and ECM remodeling: Relaxin can stimulate NO production through eNOS phosphorylation, leading to increased cGMP levels. cGMP signaling can inhibit fibroblast proliferation and promote MMP activity, contributing to ECM remodeling and potentially reducing fibrosis.

d) Smad2/3 pathway (Transforming growth factor-β (TGF-β) antagonist):

• Relaxin can interfere with TGF-β signaling, a key driver of fibrosis. It may prevent Smad2/3 phosphorylation, blocking their nuclear translocation and subsequent pro-fibrotic gene expression.

3. Integration and Context-Dependent Effects:

• These pathways don’t operate in isolation; their interplay and relative dominance in different contexts determine the overall outcome.
• Relaxin’s effects on fibroblast behavior and MMP activity can vary depending on factors like the specific kidney disease, the stage of fibrosis, and other signaling cues present in the microenvironment.

Beyond the Spotlight: Unveiling Alternative Receptors:

Alongside RXFP1, several other receptors have been implicated in relaxin’s renal effects:

• GPR107: This G protein-coupled receptor, also known as relaxin-3 receptor, interacts with relaxin-3, a variant of relaxin-2. Studies suggest GPR107 contributes to relaxin’s anti-fibrotic and anti-inflammatory properties in the kidney.

• Leucine-rich repeat-containing G protein-coupled receptors (LGRs): These receptors bind relaxin-like family peptides such as INSL5 and may influence renal function through pathways independent of RXFP1.

• Toll-like receptors (TLRs): These immune receptors recognize relaxin-2 as a danger signal and may trigger inflammatory responses in certain contexts, potentially influencing CKD progression.

Uncharted Territory: Exploring Alternative Mechanisms:

Beyond receptor-mediated signaling, several additional mechanisms contribute to relaxin’s beneficial effects in CKD:

• Direct modulation of gene expression: Relaxin can directly interact with nuclear proteins like Nrf2, influencing the expression of antioxidant and anti-fibrotic genes.

• Regulation of non-coding RNAs: MicroRNAs and long non-coding RNAs (lncRNAs) are involved in various cellular processes, and relaxin can influence their expression, impacting renal cell behavior and disease progression.

• Post-translational modifications: Protein phosphorylation, acetylation, and other modifications can alter protein function and activity. Relaxin can influence these processes, modulating the actions of key signaling molecules in the kidney.

Unveiling the Puzzle: Towards a Holistic Understanding:

Understanding the interplay between these diverse receptors and mechanisms is crucial for harnessing the full potential of relaxin in CKD therapy. Future research should delve deeper into:

• Identifying the specific contexts and cell types where different receptors and mechanisms dominate relaxin’s actions.
• Deciphering the complex crosstalk between various signaling pathways triggered by relaxin.
• Investigating the potential interplay between relaxin and other therapeutic agents for synergistic effects in CKD treatment.

By unraveling the intricate tapestry beyond RXFP1, we can move closer to unlocking the full potential of relaxin as a powerful therapeutic tool for patients suffering from chronic kidney disease.