Peptide Receptor Types: GPCRs, Ion Channels & Beyond

GPCRs are the largest family of cell surface receptors, with over 800 members in humans. They are the target for approximately 34% of all approved drugs — and the majority of peptide drugs.

Structure: - Seven transmembrane alpha-helical domains (hence "7TM receptors") - An extracellular N-terminus and intracellular C-terminus - Three extracellular loops (for ligand binding) and three intracellular loops (for G-protein coupling)

The signalling cascade: 1. Peptide binds to the receptor's extracellular domain 2. Conformational change propagates through the transmembrane domains 3. The intracellular domain activates a heterotrimeric G-protein (Gα, Gβ, Gγ) 4. The Gα subunit activates downstream effectors depending on its subtype

G-protein subtypes and their effects: - Gαs: Activates adenylyl cyclase → increases cAMP → activates PKA (e.g., GLP-1R, GHRHR) - Gαi: Inhibits adenylyl cyclase → decreases cAMP (e.g., somatostatin receptors) - Gαq: Activates phospholipase C → increases IP3 and DAG → calcium release and PKC activation (e.g., GnRH receptor)

Peptide-targeting GPCRs (examples): - GLP-1 receptor (semaglutide) - GHRH receptor (sermorelin, CJC-1295) - GHS-R1a / ghrelin receptor (ipamorelin, MK-677) - Melanocortin receptors MC1R-MC5R (melanotan II, PT-141) - Opioid receptors (endorphins, enkephalins)

The dominance of GPCRs in peptide signalling makes this receptor family essential knowledge for understanding peptide pharmacology.

While GPCRs dominate classical peptide signalling, receptor tyrosine kinases mediate the effects of several growth factor peptides.

Structure: - Single transmembrane domain - Extracellular ligand-binding domain - Intracellular tyrosine kinase domain

The signalling mechanism: 1. Growth factor peptide binds to the extracellular domain 2. Receptor dimerisation occurs (two receptor molecules come together) 3. The intracellular kinase domains cross-phosphorylate each other (trans-autophosphorylation) 4. Phosphorylated tyrosine residues create docking sites for signalling proteins 5. Multiple downstream pathways are activated simultaneously

Key downstream pathways: - Ras-MAPK pathway: Cell proliferation and differentiation - PI3K-Akt pathway: Cell survival, growth, and metabolism - PLCγ pathway: Calcium signalling - JAK-STAT pathway: Gene transcription (particularly for cytokine receptors)

Relevant peptide/growth factor ligands: - IGF-1 receptor: Mediates growth hormone's anabolic effects through IGF-1 binding - EGF receptor: Relevant to BPC-157's proposed cytoprotective mechanisms - FGF receptors: Fibroblast growth factors involved in tissue repair - VEGF receptors: Angiogenesis signalling — relevant to wound healing peptides

RTK signalling tends to be slower in onset but longer in duration compared to GPCR signalling, as it involves gene transcription changes rather than rapid second messenger cascades.

Beyond GPCRs and RTKs, peptides interact with several other receptor classes:

Ligand-gated ion channels: - Receptor opens an ion channel directly upon ligand binding - Produces the fastest cellular responses (milliseconds) - Example: GABAA receptors — while primarily responsive to GABA, certain peptides modulate their function (selank enhances GABA binding) - Nicotinic acetylcholine receptors — targeted indirectly by some neuropeptides

Intracellular receptors: - Some peptides cross the cell membrane and act on intracellular targets - Nuclear receptors: typically targeted by steroid hormones, but some peptide fragments may interact with nuclear signalling - MOTS-c translocates to the nucleus under stress conditions, interacting with transcription factors directly

Toll-like receptors (TLRs): - Pattern recognition receptors of the innate immune system - Certain antimicrobial peptides (e.g., LL-37) interact with TLRs to modulate immune responses - Thymosin alpha-1 may influence TLR signalling pathways

Receptor serine/threonine kinases: - TGF-β superfamily receptors use serine/threonine kinase activity - Relevant to tissue remodelling and fibrosis pathways - Some peptides influence TGF-β signalling indirectly

The emerging concept of biased agonism: A single receptor can activate multiple downstream pathways. "Biased agonists" preferentially activate one pathway over another. This concept is being exploited in peptide drug design — for example, developing GLP-1R agonists that favour the appetite-suppressing pathway over the nausea-inducing pathway.

A critical concept for understanding peptide dosing is receptor desensitisation — the process by which receptors become less responsive to sustained or repeated stimulation.

Mechanisms of desensitisation:

Homologous desensitisation (receptor-specific): 1. Phosphorylation: G-protein receptor kinases (GRKs) phosphorylate the activated receptor 2. Arrestin binding: Beta-arrestin binds to the phosphorylated receptor, blocking G-protein coupling 3. Internalisation: The receptor-arrestin complex is internalised via endocytosis 4. Recycling or degradation: Internalised receptors are either recycled back to the surface or degraded in lysosomes

Heterologous desensitisation: - Second messenger kinases (PKA, PKC) phosphorylate multiple receptor types simultaneously - This can dampen responses across multiple signalling pathways

Clinical relevance for peptides: - GnRH agonists: Continuous administration actually suppresses gonadotrophin release (used therapeutically in prostate cancer and endometriosis) - GH secretagogues: Prolonged continuous use may reduce GH pulse amplitude due to somatotroph desensitisation - Melanocortin agonists: Tachyphylaxis to nausea and flushing often develops, but tolerance to pigmentation effects is less pronounced

Dosing implications: - Pulsatile or intermittent dosing can maintain receptor sensitivity - "Drug holidays" or cycling protocols aim to prevent desensitisation - The half-life of the peptide determines whether exposure is pulsatile or continuous - CJC-1295-DAC's long half-life produces continuous GHRH receptor stimulation, which may cause more desensitisation than pulsatile Mod GRF 1-29

Understanding receptor pharmacology terminology is essential for interpreting peptide research:

Full agonists: - Bind the receptor and produce the maximum possible response - Example: native GLP-1 at the GLP-1 receptor

Partial agonists: - Bind the receptor but produce a submaximal response, even at full receptor occupancy - Can actually antagonise a full agonist by competing for receptor binding whilst producing a weaker signal

Antagonists: - Bind the receptor without activating it, blocking the natural ligand - Competitive antagonists can be overcome by increasing agonist concentration - Non-competitive antagonists cannot be overcome by more agonist

Inverse agonists: - Bind the receptor and produce the opposite effect to an agonist - Possible because some receptors have constitutive (baseline) activity

Allosteric modulators: - Bind to a different site on the receptor (not the primary ligand-binding site) - Positive allosteric modulators (PAMs) enhance the response to the natural ligand - Negative allosteric modulators (NAMs) reduce it - Selank may act partly as an allosteric modulator of GABAA receptors

Why this matters for peptide users: The type of receptor interaction determines a peptide's effect profile, side effects, and dosing requirements. A full agonist with a long half-life (like semaglutide) produces a very different clinical profile from a partial agonist with a short half-life, even if both target the same receptor.

*This article is for educational purposes only. Receptor pharmacology is fundamental to understanding how peptides work. Use peptides only under the guidance of a qualified healthcare professional.*