Lesson 8 - The Cell's Whisperers: Signal Transduction, Receptors, Ligands, and the Amplified Cascade

Dive into the fascinating world of cellular communication, where signal transduction ensures coordinated life processes. Signal molecules act through either membrane receptors or intracellular receptors. Membrane receptors are crucial, initiating a chain of intracellular reactions that can ultimately modify gene expression. We examine how the selective recognition of a ligand by its receptor depends on shape and amino acid residues, and how high or low affinity determines receptor occupancy.

Learn about the massive family of G-Protein Coupled Receptors (GPCRs), structurally defined by seven transmembrane α-helical domains. These receptors transmit signals using G proteins, which are divided into classes like Gs (stimulating adenylyl cyclase) and Gi (inhibiting adenylyl cyclase).

The episode explores the role of second messengers—small molecules like cAMP, Ca2+, IP3​ and DAG - which coordinate large-scale cellular responses. We detail how cAMP primarily exerts its effects via Protein Kinase A (PKA), and how signal amplification occurs through enzymatic cascades, such as the Ras-MAP kinase pathway activated by Receptor Tyrosine Kinases. We also discuss how synthetic molecules, known as agonists (like Salbutamol) and antagonists (like Cimetidine), interact with these receptors to activate or block function.

Finally, we address intracellular signaling, where small hydrophobic molecules (like steroid hormones, retinoids, and Vitamin D) diffuse through the membrane to bind to nuclear receptors, regulating gene transcription. Understanding these pathways is critical, especially when considering dysfunctions: we review how toxins from bacteria like Vibrio cholerae and Bordetella pertussis chemically modify G proteins, leading to disease.