Cell Signalling

Cell signalling can be autocrine, paracrine, endocrine, or juxtacrine. The process involves a signal (ligand), a receptor, and a transduction pathway. Receptors are either on the cell surface (GPCRs, Ion channels, Enzyme-linked) or intracellular (for lipophilic molecules like steroids). G-Protein Coupled Receptors (GPCRs) are the largest class and work via second messengers like cAMP or IP3/DAG. For example, Gs stimulates adenylyl cyclase to increase cAMP, while Gi inhibits it. Gq activates phospholipase C. Enzyme-linked receptors, such as the Insulin receptor, have intrinsic tyrosine kinase activity that triggers phosphorylation cascades (e.g., MAPK pathway). Signal transduction often involves a 'phosphorylation cascade' where protein kinases activate one another, leading to a change in gene expression or cellular function. Signal termination is just as important as initiation, achieved via phosphatases, phosphodiesterases (which break down cAMP), or ligand degradation/reuptake.

GPCRs function by exchanging GDP for GTP on the alpha subunit when a ligand binds. Gs activates Adenylyl Cyclase -> cAMP -> Protein Kinase A (PKA). Gq activates Phospholipase C -> PIP2 cleavage into IP3 and DAG. IP3 opens calcium channels, while DAG activates Protein Kinase C (PKC). Many drugs target these pathways: Sildenafil inhibits phosphodiesterase-5 (PDE5), keeping cGMP levels high and causing vasodilation. Adrenaline acts via alpha and beta-adrenergic receptors, which are all GPCRs (alpha-1 is Gq, alpha-2 is Gi, beta is Gs). In the Insulin pathway, binding to the receptor causes auto-phosphorylation of tyrosine residues, which recruits Insulin Receptor Substrate (IRS-1), leading to the PI3K/Akt pathway, which translocation of GLUT4 transporters to the membrane and increases glucose uptake. Oncogenes, like mutated RAS, code for signalling proteins that are stuck in the 'on' position, leading to uncontrolled cell growth.

Knowledge of signalling is vital for understanding endocrinology and pharmacology. Beta-blockers antagonize Gs-coupled receptors to slow heart rate. Cholera toxin works by permanently 'locking' Gs in the active state in intestinal cells, leading to high cAMP and massive secretory diarrhoea. Many modern cancer treatments are 'Tyrosine Kinase Inhibitors' (e.g., Imatinib for CML). Understanding the second messenger systems explains the side effects of drugs; for instance, many drugs cause 'off-target' effects by interacting with different G-protein pathways in various tissues. Sildenafil interaction with nitrates is life-threatening because both increase cGMP, leading to profound hypotension.

Investigations into signalling disorders often involve measuring hormone levels (e.g., TSH, Cortisol) or assessing cellular responses (e.g., Glucose Tolerance Test). In oncology, immunohistochemistry (IHC) or genetic testing is used to identify mutated signalling proteins (e.g., HER2, KRAS, BRAF) to guide targeted therapy.

Management involves replacing missing signals (e.g., Insulin for T1DM), blocking overactive signals (e.g., ACE inhibitors, Beta-blockers), or modulating second messenger levels (e.g., Theophylline for asthma by inhibiting phosphodiesterase). Targeted cancer therapies specifically inhibit mutated signalling proteins to slow tumor progression.