Protease-Activated Receptors Overview

Protease-Activated Receptors (PARs)

• PARs are a family of G protein-coupled receptors (GPCRs) activated by proteolytic cleavage.
• Proteolytic cleavage exposes a tethered ligand domain within the receptor, which then binds and activates the receptor, initiating signaling cascades.
• The cleavage is usually performed by serine proteases.
• Examples of serine proteases that activate PARs include thrombin (acts on PAR1, 3, and 4) and trypsin (acts on PAR2).
• PARs contribute to tissue responses to injury including repair, homeostasis, cell survival, inflammation, and pain transmission.
• PARs function through the actions of various pathways including Gi (cAMP inhibitory), 12/13 (Rho and Ras activation), and q (calcium signaling).
• Proteases comprise roughly 2% of the human genome, and have diverse biological functions, often including cleaving a ligand or receptor.

Orphan Receptors

• Orphan receptors are GPCRs with no identified endogenous ligand.
• There are approximately 210 known ligand receptors, and 160 receptors with unknown ligand or function.
• Half of the human genome encodes sensory receptors, with approximately 360 receptors.
• If an orphan receptor's ligand is discovered later, the receptor is termed an "adopted orphan receptor" and is usually given the GPR followed by a number (e.g., GPR1).
• Classification of known and orphan GPCRs: 39% class I known, 37% class I orphans, 6% class II orphans, and 5% class II known.

Identification of Ligands/Deorphanization

• Methods of identifying ligands for orphan receptors include:
  • Screening known ligands (e.g., from compound libraries).
  • Screening biological extracts (e.g., from biological sources).
• Methods include an assay following screening to identify potential ligands.
• "Reverse pharmacology" is a useful approach.
• Assays include calcium measurement, cAMP measurement, internalization of tagged receptors, arachidonic acid release (PLA2), and electrophysiology.

Examples of Adopted Receptors

• Examples of adopted orphan receptors (part of the GPCR family) include the histamine H4 receptor, orexin receptor, and prolactin-releasing peptide receptor.
• Examples of adopted orphan receptors (part of the nuclear receptor group) include farnesoid X receptor (FXR), liver X receptor (LXR), and peroxisome proliferator-activated receptor (PPAR).

Proteases for PARs

• Thrombin acts on PARs 1, 3, and 4; Trypsin acts on PAR 2.
• Many proteases are produced during tissue damage.
• Many proteases rely on serine residues in their catalytic domains to induce proteolysis.
• Cleavage sites often include Lysine (K) and Arginine (R).
• Examples of Proteases:
  • TACE and ACE

Example Proteases

• TACE and ACE cleave the precursors of active soluble hormones.
• Neutral endopeptidase inactivates neuropeptide substance P.
• Cleavage of special GPCRs (PARs) on cell membranes can cause receptor activation.

PAR Activation

• Thrombin binds to PAR 1/3 due to the presence of a hirudin-like site.
• It cleaves the peptide sequence of the receptor's N-terminal and allows the binding of tethered ligands to the receptor.
• Binding to one receptor can facilitate cleavage of another receptor on the same cell (e.g. PAR3 and PAR4).

PAR Internalization

• There are two types of PAR internalization: constitutive and agonist-induced internalization.
• Following activation, PARs are regulated by ubiquitin-mediated internalization and clathrin-coated pits mediated internalization.

This quiz covers the key concepts of protease-activated receptors (PARs), including their activation mechanisms, the types of proteases involved, and their biological significance. Additionally, the quiz touches on orphan receptors and their characteristics as G protein-coupled receptors. Test your knowledge on these essential components of cellular signaling!