Cell signalling

Receptor families

Water-soluble messengers (peptides, catecholamines) cannot cross the membrane, so they act on surface receptors and use second messengers. Lipid-soluble hormones (steroids, thyroid hormone) cross it and act on intracellular receptors that control genes.

How a G protein works

A G protein is a trimer (α, β, γ). At rest α binds GDP. The activated receptor makes α swap GDP for GTP; α-GTP separates and acts on its effector enzyme or channel. The α subunit's own GTPase activity hydrolyses GTP to GDP, switching the signal off. Toxins that stop this switching keep pathways permanently on or off.

The three main G proteins

• Glucagon and β agonists act through Gs and cAMP: glucagon raises hepatic cAMP to stimulate glycogenolysis and gluconeogenesis; β2 agonists relax bronchial smooth muscle.
• Vasoconstriction by the IP₃ mechanism is produced by α1 agonists (noradrenaline, phenylephrine), angiotensin II, vasopressin on V1 and endothelin.
• Paired receptors often use opposite G proteins: α1 Gq but α2 Gi; M3 Gq but M2 Gi; V1 Gq but V2 Gs; D1 Gs but D2 Gi.

Second messengers

• cAMP: made from ATP by adenylyl cyclase; activates protein kinase A, which phosphorylates target proteins; destroyed by phosphodiesterase. Caffeine, theophylline and milrinone inhibit PDE and raise cAMP.
• IP₃ and DAG: phospholipase C splits membrane PIP₂ into IP₃, which opens IP₃ receptors on the endoplasmic reticulum to release Ca²⁺, and DAG, which with Ca²⁺ activates protein kinase C. Phospholipase C is the effector enzyme, not a messenger itself.
• Ca²⁺–calmodulin: activates myosin light chain kinase, CaM kinases and nitric oxide synthase.
• cGMP: made by soluble guanylate cyclase when nitric oxide binds it (nitrates, endothelial NO) or by membrane guanylate cyclase when ANP or BNP bind. cGMP activates protein kinase G, lowering Ca²⁺ and relaxing smooth muscle. PDE5 breaks it down; sildenafil inhibits PDE5 and prolongs erection, so it must not be combined with nitrates (severe hypotension).

Bacterial toxins and G proteins

• Cholera toxin ADP-ribosylates Gsα and blocks its GTPase, locking adenylyl cyclase on. Intestinal cAMP rises, CFTR opens and chloride and water pour into the lumen: profuse secretory diarrhoea. Heat-labile *E. coli* toxin works the same way.
• Pertussis toxin ADP-ribosylates Giα, preventing inhibition of adenylyl cyclase, so cAMP also rises (lymphocytosis, insulin release).

Receptor tyrosine kinases and JAK–STAT

• Insulin binds a pre-formed dimeric receptor; autophosphorylation recruits insulin receptor substrates, activating PI3K–Akt (GLUT4 insertion in muscle and fat, glycogen synthesis) and Ras–MAPK (growth). Growth factor receptors dimerise on binding.
• Oncogenic mutations of this pathway (HER2, EGFR, RAS, BRAF, BCR-ABL) drive cancers and are drug targets (trastuzumab, gefitinib, vemurafenib, imatinib).
• Cytokine-type receptors (GH, prolactin, EPO, leptin, interferons) lack kinase activity and use Janus kinases, which phosphorylate STAT transcription factors. JAK2 V617F causes polycythaemia vera and other myeloproliferative neoplasms.

Intracellular receptors

Steroids and thyroid hormone bind receptors that are zinc-finger transcription factors. Glucocorticoid receptors sit in the cytoplasm bound to heat-shock proteins and move to the nucleus after binding hormone; thyroid hormone receptors sit on DNA even without hormone. The effect needs new mRNA and protein, so onset is slow (hours) and duration long.

Receptor regulation

• Desensitisation: continued agonist exposure leads to receptor phosphorylation and binding of β-arrestin, uncoupling the G protein (tolerance to β agonists and nitrates).
• Down-regulation: receptors are internalised and degraded, reducing their number (insulin resistance with hyperinsulinaemia).
• Up-regulation: more receptors after denervation or chronic blockade, explaining rebound tachycardia after stopping β-blockers and denervation supersensitivity.

Remember Gs raises cAMP, Gi lowers it, Gq splits PIP₂ into IP₃ and DAG; NO and ANP make cGMP.

Exam trap V1 is Gq (vasoconstriction); V2 is Gs (aquaporin-2 insertion in the collecting duct).

Exam trap Thyroid hormone works through a nuclear receptor like steroids, despite being an amino acid derivative.

How FCPS asks about this

• Glucagon and β agonists: Gs; raise cAMP, which activates protein kinase A.
• Hormones using IP₃: α1 agonists, V1 vasopressin, angiotensin II, oxytocin, GnRH, TRH; phospholipase C generates IP₃ and DAG.
• IP₃ mechanism: releases Ca²⁺ from the endoplasmic reticulum.
• cGMP is the second messenger of: nitric oxide (soluble guanylate cyclase) and ANP or BNP (membrane guanylate cyclase).
• Cholera toxin: permanently activates Gs; pertussis toxin inactivates Gi.
• Insulin receptor: tyrosine kinase; GH and prolactin use JAK–STAT.
• Steroid and thyroid hormones: intracellular receptors that change gene transcription.