Cell Signalling & Receptors

The human body contains approximately 37 trillion cells — all derived from a single fertilised egg, all carrying the same DNA, yet forming hundreds of distinct cell types that perform coordinated functions. This coordination requires cell signalling — the process by which cells receive, process, and respond to chemical messages from other cells or from the environment. What signals do: - Instruct cells to divide, differentiate, or die (apoptosis) - Coordinate metabolism (e.g. insulin telling cells to take up glucose) - Generate immune responses - Control hormone release - Regulate heartbeat, blood pressure, and breathing - Mediate sensation and thought The basic principle of cell signalling: A signalling molecule (ligand) binds to a specific receptor on or in the target cell → the receptor changes shape → this triggers a cascade of intracellular events → the cell responds. Three key requirements: 1. Specificity — only cells with the right receptor respond to a particular signal. A liver cell and a muscle cell both receive the same insulin signal, but respond differently (liver stores glycogen; muscle takes up glucose for fuel) 2. Amplification — one signalling molecule binding one receptor can trigger thousands of downstream reactions (via signal cascades). This means tiny concentrations of hormones can have enormous effects 3. Termination — signals must be switched off to allow precise control. Receptors are internalised (endocytosis), ligands degraded, and downstream signalling molecules inactivated The range of signalling: - Endocrine — hormone released into blood, acts on distant target cells (long range, slow, persistent) - Paracrine — signal acts on neighbouring cells (e.g. growth factors in wound healing) - Autocrine — cell signals to itself (common in cancer cells that provide their own growth signals) - Synaptic — neurotransmitter released into synaptic cleft, acts on adjacent post-synaptic cell (fast, targeted)

G protein-coupled receptors (GPCRs) are the largest family of cell surface receptors — with over 800 members in humans. They mediate responses to an enormous range of signals: hormones, neurotransmitters, light, odorants (smell), and many drugs. Approximately 30–40% of all prescription drugs target GPCRs. Structure: GPCRs have 7 transmembrane domains (helices that span the cell membrane 7 times) — hence they are also called "7-TM receptors." The ligand binds on the extracellular side; the intracellular face couples to a G protein (guanine nucleotide-binding protein). How GPCRs work — the cAMP pathway: 1. Ligand binds GPCR → receptor changes shape 2. Receptor activates G protein → alpha subunit exchanges GDP for GTP and dissociates 3. Alpha subunit activates adenylyl cyclase 4. Adenylyl cyclase converts ATP → cAMP (cyclic AMP) — the "second messenger" 5. cAMP activates Protein Kinase A (PKA) 6. PKA phosphorylates target proteins → cell responds (e.g. glycogen breakdown, gene expression changes) 7. Phosphodiesterase degrades cAMP → signal terminated Examples of GPCR-mediated responses: - Adrenaline on heart muscle (via beta-1 adrenoceptors → cAMP → PKA → increased heart rate and force) - Glucagon on liver (via glucagon receptor → cAMP → PKA → activates glycogen phosphorylase → glycogenolysis) - TSH on thyroid (→ cAMP → thyroid hormone synthesis) - Morphine on CNS (via mu-opioid receptors, GPCRs that decrease cAMP → pain relief) Drug targets: - Beta-blockers (e.g. propranolol) — block beta-adrenoceptors (GPCRs) → lower heart rate and blood pressure - Salbutamol — activates beta-2 adrenoceptors (GPCRs) on bronchiole smooth muscle → bronchodilation in asthma - Antihistamines — block H1 histamine receptors (GPCRs) → reduce allergic responses

Receptor tyrosine kinases (RTKs) are a family of cell surface receptors that have intrinsic enzymatic activity — they can directly phosphorylate tyrosine residues on themselves and downstream signalling proteins. They are the primary receptors for growth factors — signals that promote cell growth and division. Structure: RTKs have: - An extracellular ligand-binding domain - A single transmembrane domain - An intracellular tyrosine kinase domain How RTKs work: 1. Growth factor binds to RTK → two RTK molecules come together (dimerisation) 2. Dimerisation activates the kinase domains → they phosphorylate each other (trans-autophosphorylation) 3. Phosphorylated tyrosines recruit intracellular signalling proteins (via SH2 domains) → activating downstream cascades: - RAS-MAPK pathway → gene expression → cell proliferation - PI3K-AKT pathway → cell survival and growth 4. Signal is terminated by phosphatases (remove phosphate groups) and receptor endocytosis Examples: - Insulin receptor — an RTK. Insulin binds → receptor autophosphorylates → activates PI3K-AKT → GLUT4 moves to cell surface → glucose uptake - EGFR (Epidermal Growth Factor Receptor) — promotes epithelial cell growth - HER2 — an RTK overexpressed in ~20% of breast cancers → drives uncontrolled growth RTKs and cancer: Growth factor receptors and their downstream pathways are among the most frequently mutated proteins in cancer. Mutations can cause RTKs to be: - Constitutively active (always switched on without ligand) → unstoppable growth signal - Overexpressed (too many copies) → hypersensitive to growth signals Targeted cancer drugs (RTK inhibitors): - Imatinib (Gleevec) — inhibits BCR-ABL, a constitutively active tyrosine kinase in CML. Transformed a previously fatal cancer into a manageable chronic condition - Trastuzumab (Herceptin) — monoclonal antibody against HER2 → blocks growth signal in HER2-positive breast cancer - Erlotinib, gefitinib — EGFR inhibitors used in lung cancer

Signals from surface receptors must be relayed to the nucleus (to change gene expression) and to cytoplasmic proteins (to change cell behaviour). This is done through signalling cascades — sequences of proteins that sequentially activate each other, often through phosphorylation. The RAS-MAPK cascade: One of the most important growth-promoting pathways in the cell. Activated by many RTKs and GPCRs: 1. Activated receptor → recruits GRB2 adaptor protein → recruits SOS (a GEF, guanine nucleotide exchange factor) 2. SOS activates RAS (a small GTPase — similar to the G protein in GPCRs) → RAS-GTP is active 3. Active RAS activates RAF (a serine/threonine kinase) 4. RAF → phosphorylates and activates MEK → MEK phosphorylates and activates ERK 5. ERK enters the nucleus → phosphorylates transcription factors → switches on genes for cell growth and division 6. Termination: RAS converts GTP → GDP (intrinsic GTPase activity) → switches off RAS mutations and cancer: RAS is the most frequently mutated oncogene in human cancer (~30% of all cancers). The common mutation (e.g. G12V) impairs GTPase activity → RAS is stuck in the "on" position (permanently GTP-bound) → constitutive growth signalling → uncontrolled cell division. - KRAS mutations: most common in pancreatic (90%), lung, and colorectal cancer - HRAS mutations: bladder cancer - NRAS mutations: melanoma, leukaemia The PI3K-AKT-mTOR pathway: Another key growth and survival pathway: 1. RTK → activates PI3K (phosphoinositide 3-kinase) → generates PIP3 in the membrane 2. PIP3 recruits AKT to the membrane → AKT is phosphorylated and activated 3. AKT phosphorylates many targets: promotes cell survival (inhibits apoptosis), activates mTOR (promotes protein synthesis and growth) 4. Terminated by PTEN (phosphatase — removes the PIP3 signal) PTEN is the second most commonly lost tumour suppressor in cancer — when PTEN is lost, AKT and mTOR are constitutively active → cell cannot die and grows constantly.

Apoptosis (from the Greek for "falling off" — like leaves from a tree) is a highly organised, programmed form of cell death that is essential for development, immune function, and cancer prevention. Unlike necrosis (accidental cell death from injury), apoptosis is an active, energy-requiring process that is deliberately executed by the cell. Why apoptosis is essential: - During development: sculpts structures (fingers are separated because the cells between them apoptose), removes excess neurons (only neurons that make proper connections survive) - Immune system: kills self-reactive T cells (preventing autoimmunity); kills virus-infected cells - Tissue homeostasis: maintains organ size — cells that divide must be balanced by cells that die - Cancer prevention: cells that sustain DNA damage are instructed to apoptose rather than pass mutations on The molecular machinery: Apoptosis is executed by caspases — a family of cysteine proteases that cleave proteins at aspartate residues. They exist as inactive precursors (pro-caspases) and are activated in a cascade. Two main pathways: Intrinsic pathway (internal stress signals — DNA damage, oxidative stress, growth factor withdrawal): - Pro-apoptotic proteins (BAX, BAK) form pores in the mitochondrial outer membrane - Cytochrome c is released from mitochondria → forms the apoptosome → activates caspase-9 → activates executioner caspases (3, 7) → cell is dismantled - Anti-apoptotic proteins (BCL-2, BCL-XL) block BAX/BAK — overexpression of BCL-2 in cancer cells prevents apoptosis → cells accumulate Extrinsic pathway (external "death signals"): - FasL or TRAIL binds death receptors (Fas/DR5) → activates caspase-8 → activates executioner caspases Apoptosis and cancer: Cancer cells find multiple ways to evade apoptosis: - Overexpression of anti-apoptotic BCL-2 proteins (targeted by venetoclax — a BCL-2 inhibitor used in leukaemia) - Loss of p53 (the "guardian of the genome" — p53 is mutated in 50% of all cancers; it normally triggers apoptosis in response to DNA damage) - Activation of the PI3K-AKT survival pathway p53 — the guardian of the genome: p53 is a transcription factor activated by DNA damage, hypoxia, and oncogene activation. When activated it: arrests the cell cycle (to allow DNA repair), activates DNA repair genes, and if damage is irreparable, triggers apoptosis. Loss of p53 function is the most common single event in cancer development.