Membrane-bound receptors and signal transduction

• Receptors
• On cell surface. Anchored in plasma membrane
• Ionotropic receptor (With integral ion channel function)
• G protein coupled receptors
• Receptors with integral enzymatic function
• Growth factors, Insulin, ANP
• In cytoplasma
• Steroid hormones. Lipophilic substances
• Phosphorylation
• Effector proteins
• Serine. Threonine.
• Intracellular messengers
• Kinase enzymes
• Protein kinase A (cAMP)
• Protein kinase G (cGMP)
• Protein kinase C (diacylglycerol)
• Activation
• Ion channels. Enzymes. Transporter

IONOTROPIC RECEPTORS

• e.g. nAChR, GABAa, 5HT3
• Rapid (milisecond) opening of integral receptor ion channel
• Changes in membrane potential
• Biological response
• Neurotransmission. Muscle transmission

G PROTEIN-COUPLED RECEPTORS

• e.g. mAChR, alpha and beta adrenoreceptors, dopamine, 5HT, opiate, peptides
• Extracellular N-terminus. Intracellular C-terminus. 7-membrane spanning domains
• Ligand binds to domain (extracellular)
• Conformational change
• Guanine nucleotide-binding  (G) protein binds with cytoplasmic domain
• Families characterised by relatedness of alpha subunits
• Gi
• Alpha i
• Inhibits adenylate cyclase
• Activates K channels
• Gq
• Activates phospholipase C
• Catalyse formation of DAG, IP3
• Signal transduction
• Intracellular messengers (proteins)
• cAMP
• Phosphorylation of serine and threonine
• Activates protein kinase A
• Inositol 1,4,5-triphosphate (IP3)
• Binds to receptor on endoplasmic reticulum
• Release of stored Ca ions
• Intracellular signal
• Diacylglycerol (DAG)
• Diffuses freely
• Activates protein kinase C (PKC)
• Malfunctions
• Cholera
• Cholera toxins catalysing ADP-ribosylation of alpha s subunit of Gs in enterocytes
• Gs unable to hydrolyse guanosine triphosphate (GTP)
• GTP needed to terminate Gs activation
• High level cAMP. High Na and H2O efflux into lumen. Watery diarrhoea. 

RECEPTORS WITH INTEGRAL ENZYMATIC FUNCTION

• Tyrosine kinase-linked receptors
• Insulin, growth factors, receptor guanylate cyclases (receptors for natriuretic peptides)
• Large extracellular N-terminal (ligand binding). Membrane spanning helix. Intracellular C-terminal (enzymatic activity)

Receptor tyrosine kinases

• Ligand + receptor tyrosine kinases ￫ conformational change ￫ receptor dimerisation ￫ Activation integral tyrosine kinase activity ￫ Cytoplasmic domain ￫Autophosphorylation ￫ Expose binding site to cytosolic protein with SH2 (src homology 2) domain
• Phosphatidylinositol 3-kinase, GTPase-activating protein, Phospholipase C-gamma
• Intracellular processes
• Enzyme activation & alterations in gene transcription
• Overactive receptor tyrosine kinase
• Multiple endocrine neoplasia type 2
• Medullary thyroid carcioma, phaeochromocytoma, hyperparathyroidism
• Mutations in the RET gene on chromosome 10
• Protein product: Membrane-bound tyrosine kinase
• Constitutive receptor activation ￫ Unchecked growth signal, tumour formation
• Proto-oncogenes
• Encode components of signal transduction pathways
• Mutations ￫ cancers
• Monoclonal recombinant antibodies
• Trastuzamab vs HER2
• Invasive breast cancers 
• 25% over express the epidermal growth factor receptor tyrosine kinase, HER2
• Adverse prognosis
• Erlotinib, Geftinib vs different class epidermal growth factor receptor
• Non-small cell lung cancer

Receptor guanylate cyclases

• Receptors for atrial natriuretic peptides (ANPs)
• Ligand + Extracellular domain of receptor ￫ Guanylate cyclase activity ￫ Activation of intracellular domain ￫ GTP ￫ cGMP ￫ Protein kinase G ￫ Phosphorylates & activates intracellular effector proteins ￫ Biological response
• Nitric oxide
• Paracrine mediator
• From endothelial cells, some neurones and inflammatory cells
• Lacks classical receptor
• Unique mechanism of action
• NO ￫ Diffuses freely into cells ￫ Activates soluble cytosolic form of guanylate cyclase ￫ Rise in cGMP
• Exogenous NO
• Nitrovasodilator drugs 
• Glyceryl trinitrate, Sodium nitroprusside
• Rise in cGMP in vascular smooth muscle cells ￫ Relaxation of blood vessels