Receptors

• Receptors are responsible for receiving chemical messages

Receptors for Chemical Messengers

• Inside the cell (nucleus or cytosol) – substances soluble in fat, products of intracellular metabolism, such as steroid hormones, T-hormones, fatty acids, cholesterol, and prostaglandins
• On the cell membrane – lipid insoluble substances, such as amines, amino acids, peptides, eikosanoids, and purines

Receptor Properties

• Specificity: Receptor (RC) binds only one type of messenger (or structurally related ones)
• Affinity: Strength with which a messenger binds to RC
• Saturation: Degree to which RC is occupied by a messenger

Intracellular Receptors

• Cytoplasmic receptors include steroid hormones, vitamin D, and prostaglandins.
• Nuclear receptors include T-hormones

Cytoplasmic Receptors

• Ligand diffusion across the membrane
• Binding of the ligand to its receptor
• Dissociation of the accompanying protein (chaperone)
• Dimerization of complexes R-H
• Transport of the complex R-H to the nucleus
• Binding of R-H on the HRE sequence of DNA (hormone response element) together with coactivators and RNA-polymerase
• Activation of transcription → synthesis of mRNA
• mRNA diffusion into the cytoplasm
• Translation activation
• Protein synthesis

Nuclear Receptors

• Ligand diffusion across the membrane
• Binding of the ligand to its nuclear receptor in a heterodimer with RXR (retinoid X receptor) – bound to HRE
• Release of corepressor and binding of coactivator and RNA-polymerase on the complex
• Activation of transcription → synthesis of mRNA
• mRNA diffusion into the cytoplasm
• Initiation of translation
• Protein synthesis

Membrane Receptors

• Extracellular messages should be transferred into the intracellular environment – diffusion (Ca2+) or synthesis of the second messenger
• Second messengers mediate immediate intracellular events or induce long-term changes.
• Kinases mediate many of these processes through phosphorylating target proteins (pumps, enzymes, channels, transcription factors).

Second Messengers

• Small diffusible molecules enable the transfer of information from the ECF into the ICF

Types of Second Messengers

• Cyclic nucleotides: cyclic adenosine monophosphate – cAMP – NE, EPI, ADH; cyclic guanosine monophosphate – cGMP – ANP
• Derivatives of membrane lipids: diacylglycerol, inositol trisphosphate (NE, ACh, ADH)
• Ca2+
• Gases (NO/CO/H2S), ROS/RNS

Second Messengers Table

• cAMP; Adenylyl cyclase (AC); protein kinase A (PKA); phosphodiesterases (PDE)
• cGMP; Guanylyl cyclase (GC); protein kinase G (PKG); phosphodiesterases (PDE)
• (Ca2+); Release from ER after IP3 stimulation; calmodulin; Uptake into ER via Ca2+ ATP-ase
• inositol-1,4,5-trisphosphate (IP3); phosphatidylinositol-4,5-bisphosphate (PIP2); phospholipase C (PLC); protein kinase C (PKC); phosphatases – dephosphorylation to inositol
• diacylglycerol (DAG); phosphatidylinositol-4,5-bisphosphate (PIP2); phospholipase C (PLC); protein kinase C (PKC); lipases – production of glycerol and FFA
• phosphatidylinosit ol-3,4,5-trisphosphate (PIP3); phosphatidylinositol-4,5-bisphosphate (PIP2); phosphatidylinositol-3-kinase (PI3K); protein kinase B (PKB); phosphatase PTEN – release of phosphate in position 3

Membrane Chemoreceptors

• Receptor channels
• G-protein coupled receptors
• Catalytic receptors

Receptor Channels, Ligand-Gated Channels

• Ligand binding to the receptor results in a conformational change in the receptor molecule
• This opens the channel, allowing ion diffusion
• Change in MP (IPSP, EPSP)
• Cellular response
• Receptors include GABA (IPSP), glycine (IPSP), glutamic acid (EPSP), Ach (N) (EPSP), serotonin (EPSP), and ATP (EPSP).

G-Protein Coupled Receptors

• G-proteins are GTP-binding proteins with 3 subunits (α, β, and γ), and α subunits that display GTP-ase activity to regulate synthesis of the second messenger
• Types of G-proteins: Gas, Gai, Gaq.
• After GTP binding, βγ subunits dissociate from the G-protein, and processes lead to formation of the activated second messenger

G-Protein Coupled Receptors Action

• Ligand binds to its receptor
• The receptor-ligand complex binds to a G-protein
• G-protein exchanges GDP for GTP
• GTP associated with the G-protein dissociates into subunits α and βγ
• Subunit α activates an enzyme, producing a second messenger. βγ subunits also activate or inhibit membrane proteins (channels, enzymes)
• After GTP splitting by α subunit, complex α-GDP dissociates from the enzyme
• The second messenger is degraded by an enzyme

G Proteins acting via Adenylyl Cyclase

• Adenylyl cyclase (AC) converts ATP into cAMP
• cAMP activates protein kinase A
• PKA phosphorylates downstream proteins.
• cAMP is degraded by phosphodiesterase (PDE) to inactive AMP

G Proteins acting via Phospholipase C

• Phospholipase C (PLC) splits phosphatidylinositol-4,5-bisphospate (PIP2) from the membrane (on phosphodiesterase bond between glycerol and phosphate group)
• Two second messengers (inositol-1,4,5-triphosphate (IP3) and 1,2-diacylglycerol (DAG)) are produced
• DAG activates protein kinase C (PKC), and IP3 releases Ca2+ from the ER

G Proteins acting via Phosphodiesterase

• cGMP is bound to the inner opening of the Na+ channel in the membrane of photoreceptors
• Photoreceptor activation by light leads to stimulation of phosphodiesterase (PDE), cGMP degradation, and closing of the channel
• Decreased flow of cations into the cell (Na channel closed) results in imbalance with relatively higher outflow of cations from the cell (K+ background channels) and membrane hyperpolarization - receptor potential.

G Proteins acting via Phospholipase A2

• Increased intracellular Ca2+ leads to Cyclooxygenase which form Prostaglandins like Vascular effects and Inflammation
• Increased intracellular Ca2+ leads to Lipoxygenase which form Leukotriens like Hypersensitivity and Inflammation
• Cyclooxygenase also produce Tromboxans like Vascular effects and Hemostasis

Regulation of Smooth Muscle Contraction

• Ca++; L-type Ca++ channel cause increased SR Ca+ which releases IP3
• Increased Ca++/CM complex
• GS-R cause ADR
• MLC ATP cause increased MLC
• Guanylate cyclase cause increased CGMP via NO, leading to Relaxation

Catalytic Receptors

• Enzymes or enzyme complexes
• Mostly individual transmembrane proteins
• Frequently form dimers or oligomers after activation
• Differentiation, proliferation, and maturation; vs. Apoptosis, necrosis, and inhibition of growth and maturation

Types of Catalytic Receptors

• Guanylate cyclase
• Serine/threonine kinases
• Tyrosine kinases
• Receptors coupled to tyrosine kinases
• Phosphotyrosine phosphatases

Guanylate Cyclases

• Natriuretic peptides – ANP, BNP, and CNP
• ANP is released from atrial cardiomyocytes during atrial distension, causing vasodilation and natriuresis (decrease in BP)
• BNP is isolated from the porcine brain, but occurs in the heart ventricles and is a marker of heart failure
• CNP is isolated from the porcine brain and causes vasodilation
• The receptor directly catalyses the conversion of GTP to cGMP

Guanylyl Cyclases (GCR)

• There are at least 7 GCRs
• They regulate blood pressure and volume, inhibit cardiomyocyte growth, and are involved in water and ion transport, ossification, and pheromone detection.

Serine-Threonine Kinases

• First cloned in 1991
• Ligands include activins, inhibins, myostatin, bone morphogenetic protein, and TGF-β
• The receptor phosphorylates itself on Ser and Thr residues

Functions of Ser/Thr Kinases (STK)

• They mediate muscle growth and development, bone development, and regulation of hormone secretion
• They are also involved in proliferation, angiogenesis, and apoptosis
• SMAD (transcription activator)
• Regulation of gene expression

Tyrosine Kinases

• Major ligands include insulin, nerve growth factors (NGF, neurotrophins), and epidermal growth factor (EGF)
• The receptor phosphorylates itself on Tyr residues

Insulin

• α
• α
• β
• β

Insulin and what it does

• intermediary metabolism
• growth
• ion balance

Neurotrophic Growth Factors

• NGF (1951, isolation 1956, 1986 NP)
• Essential for neuronal growth and survival
• Secreted by the target organ

Receptors Coupled to Tyrosine Kinases

• Ligands include prolactin and other hormones (growth hormone, erythropoietin), macromolecules or products of microorganisms (PAMP - pathogen-associated molecular patterns) - Toll-like receptors, and tumor necrosis factor (TNF-α)
• Kinases JANUS/ transcription factors STAT (Signal Transducer and Activator of Transcription)
• The receptor is coupled to tyrosine kinase, which phosphorylates it

Growth Hormone (GH)

• Growth
• Intermediary metabolism
• Ionic balance

Phosphotyrosine Phosphatases

• Mainly after 2000
• Ligands include hyaluronic acid, proteins of the extracellular matrix, and other frequently unknown ligands
• The intracellular substrate is a kinase, which is prepared to activation by dephosphorylation
• The receptor removes phosphate from other proteins

CD44

• Cell aggregation and migration
• Lymphocyte activation
• Degradation of hyaluronic acid
• Hemopoiesis
• Angiogenesis
• Cytokine secretion

Down- and Up-Regulation

• Down-regulation: ↓ in total number of receptors for a given ligand with a response to a chronic ↑ in EC concentration of the ligand
• Up-regulation: ↑ in total number of receptors for a given ligand with a response to a chronic ↓ in EC concentration of the messenger

Internalization

• Receptor-mediated endocytosis
• Number of receptors decreases
• Down-regulation or recycling of receptors

Desensitization

• ↓ receptor response to messenger
• Due to high concentration or prolonged exposure to a signalling molecule
• The receptor is uncoupled from its signalling cascade