Premed · Premed · General Biology 1
Lecture 14: Cell Signaling and Signal Transduction
General Biology I — Molecular & Cellular
Learning Objectives
By the end of this lecture, students will be able to:
- Describe the general stages of cell signaling: reception, transduction, and response
- Classify types of cell signaling based on distance (autocrine, paracrine, endocrine, juxtacrine)
- Explain how G protein-coupled receptors (GPCRs) and receptor tyrosine kinases (RTKs) initiate signaling cascades
- Describe the role of second messengers (cAMP, IP3, DAG, Ca2+) in signal amplification
- Explain how signaling pathways are terminated and how defects lead to disease
Lecture Content
I. Overview of Cell Signaling
In multicellular organisms, cells must communicate with one another to coordinate growth, metabolism, differentiation, and defense. Cell signaling follows a universal three-stage logic. During reception, a signaling molecule called a ligand binds to a specific receptor on or within the target cell. During transduction, the signal is relayed and amplified through a cascade of intracellular molecular events. Finally, during the response, the cell changes its behavior--altering gene expression, adjusting metabolic activity, changing shape, initiating movement, or committing to division or death.
II. Types of Cell Signaling
Signaling is classified by the distance between the signaling cell and its target. In autocrine signaling, a cell secretes a ligand that acts on its own receptors--a strategy common among immune cells. In paracrine signaling, the signal diffuses locally to act on nearby cells, as occurs with neurotransmitters at synapses and growth factors during embryonic development. Endocrine signaling operates over long distances: hormones are released into the bloodstream and travel throughout the body to reach distant target cells--insulin secreted by the pancreas acting on liver and muscle cells is a classic example. Juxtacrine (contact-dependent) signaling requires direct cell-to-cell contact, with a membrane-bound ligand on one cell binding a receptor on an adjacent cell; the Notch signaling pathway operates this way. Synaptic signaling is a specialized form of paracrine signaling in which neurotransmitters are released into the narrow synaptic cleft between neurons. Cells can also communicate directly by passing small molecules through gap junctions (in animals) or plasmodesmata (in plants).
III. Signal Reception
Ligands are chemically diverse: they include proteins, peptides, amino acid derivatives, steroid hormones, and even gases like nitric oxide (NO). Where a receptor is located depends on the nature of its ligand.
A. Intracellular Receptors
Intracellular receptors reside in the cytoplasm or nucleus and bind hydrophobic ligands that can cross the plasma membrane unaided. Steroid hormones (estrogen, testosterone, cortisol), thyroid hormones, and nitric oxide all signal through intracellular receptors. These receptors often function as transcription factors: upon binding their ligand, they enter the nucleus (if not already there), bind to specific DNA sequences, and directly regulate gene expression. Because this pathway involves transcription and translation, the cellular response is typically slow (taking hours) but long-lasting.
B. Cell-Surface Receptors
Cell-surface receptors are transmembrane proteins that bind hydrophilic ligands unable to cross the lipid bilayer. They transduce the signal across the membrane without the ligand ever entering the cell. Three major types dominate: G protein-coupled receptors (GPCRs), receptor tyrosine kinases (RTKs), and ion channel receptors (ligand-gated ion channels).
IV. G Protein-Coupled Receptors (GPCRs)
GPCRs constitute the largest family of cell-surface receptors, with approximately 800 members encoded in the human genome. Each GPCR threads through the membrane seven times (hence "seven-transmembrane" or 7TM receptors). When a ligand binds the extracellular domain, the receptor undergoes a conformational change that activates an associated G protein on the cytoplasmic side. G proteins are trimeric GTPases composed of alpha, beta, and gamma subunits. In the inactive state, the alpha subunit binds GDP and remains associated with the beta-gamma complex. Upon receptor activation, the alpha subunit exchanges GDP for GTP, causing the trimer to dissociate into G-alpha-GTP and a free beta-gamma dimer, both of which can activate downstream effector enzymes. The alpha subunit possesses intrinsic GTPase activity and eventually hydrolyzes GTP back to GDP, returning itself to the inactive state--a built-in off switch.
Two major effector pathways branch from GPCRs. Stimulatory G proteins (Gs) activate adenylyl cyclase, which converts ATP into the second messenger cAMP, while inhibitory G proteins (Gi) suppress it. Alternatively, Gq proteins activate phospholipase C, which cleaves the membrane phospholipid PIP2 into two second messengers: IP3 and DAG. The range of ligands that signal through GPCRs is staggering: epinephrine, serotonin, dopamine, histamine, opioids, odorant molecules, and even light (via rhodopsin in the retina).
<image>A diagram showing GPCR signaling. Left panel: A GPCR with seven transmembrane domains is shown in the membrane. A ligand binds the extracellular domain, causing a conformational change. The intracellular side shows a trimeric G protein (alpha-beta-gamma) with GDP bound to alpha. Upon activation, GDP is exchanged for GTP, and alpha-GTP dissociates from beta-gamma. Right panel: Two downstream pathways — (1) Gs-alpha activates adenylyl cyclase, which converts ATP to cAMP; cAMP activates protein kinase A (PKA); (2) Gq-alpha activates phospholipase C, which cleaves PIP2 into IP3 (releases Ca2+ from ER) and DAG (activates protein kinase C, PKC). Arrows show signal amplification at each step.</image>
V. Receptor Tyrosine Kinases (RTKs)
Receptor tyrosine kinases are single-pass transmembrane proteins with intracellular kinase domains--enzymes that transfer phosphate groups from ATP to tyrosine residues on target proteins. When a ligand (typically a growth factor) binds, it induces receptor dimerization: two receptor monomers come together, and their kinase domains phosphorylate tyrosine residues on each other (cross-phosphorylation or autophosphorylation). The resulting phosphotyrosines create docking sites for downstream signaling proteins containing SH2 domains, which bind phosphotyrosine specifically.
A single activated RTK can simultaneously initiate multiple signaling cascades. The Ras-MAPK pathway drives cell growth, proliferation, and differentiation. The PI3K-Akt pathway promotes cell survival and suppresses apoptosis. The PLCgamma pathway triggers calcium signaling. RTK ligands include EGF, PDGF, FGF, insulin, and VEGF. The clinical significance of RTKs is immense: many are proto-oncogenes, and mutations that cause constitutive (ligand-independent) activation drive cancer. HER2 overexpression promotes breast cancer, EGFR mutations drive certain lung cancers, and the BCR-ABL fusion protein causes chronic myeloid leukemia. Targeted therapies such as imatinib (Gleevec) and trastuzumab (Herceptin) have transformed cancer treatment by specifically inhibiting these aberrant RTKs.
VI. Second Messengers
Second messengers are small, rapidly diffusible intracellular molecules that amplify and propagate signals far beyond the initial receptor-ligand interaction.
Cyclic AMP (cAMP) is produced by adenylyl cyclase from ATP and activates protein kinase A (PKA), which phosphorylates a wide array of target proteins to alter their activity. cAMP is degraded by phosphodiesterase (PDE), ensuring that the signal is transient. The classic cAMP cascade--epinephrine binding a GPCR, activating Gs, stimulating adenylyl cyclase, raising cAMP, activating PKA, and triggering glycogen breakdown--illustrates how a single hormone can rapidly mobilize energy stores.
IP3 and DAG are produced simultaneously when phospholipase C cleaves PIP2. IP3 diffuses to the endoplasmic reticulum and opens Ca2+ channels, flooding the cytoplasm with calcium. DAG remains in the membrane and activates protein kinase C (PKC).
Calcium ions (Ca2+) are themselves powerful second messengers. The cytoplasmic calcium concentration is normally kept extremely low (~100 nM) relative to the ER lumen and the extracellular space (~mM). When IP3-gated channels or plasma membrane channels open, the resulting surge in cytoplasmic Ca2+ activates calmodulin, a calcium-binding protein that in turn activates various kinases and enzymes. Calcium signaling controls processes as diverse as muscle contraction, neurotransmitter release, fertilization, and apoptosis.
VII. Signal Amplification
One of the most remarkable features of signaling cascades is their capacity for exponential amplification. A single ligand-receptor interaction can activate multiple G proteins. Each G protein activates one enzyme, which generates many second messenger molecules. Each second messenger activates multiple kinases, each of which phosphorylates many target proteins. Through a kinase cascade (e.g., MAPKKK -> MAPKK -> MAPK), the signal is amplified at every step. The net result is staggering: a single molecule of epinephrine can ultimately trigger the release of approximately 10^8 molecules of glucose from glycogen.
VIII. Signal Termination
Cells must be able to turn signals off as efficiently as they turn them on; otherwise, continuous stimulation would be just as harmful as no signal at all. Multiple mechanisms ensure termination. Ligands are degraded enzymatically or removed by reuptake. Receptors are inactivated through desensitization (conformational changes that prevent further signaling), internalization by receptor-mediated endocytosis, or degradation. G proteins self-inactivate through their intrinsic GTPase activity, hydrolyzing GTP to GDP. Phosphatases reverse the work of kinases by removing phosphate groups from target proteins. Phosphodiesterases break down cAMP and cGMP. Calcium pumps in the ER membrane and plasma membrane rapidly restore the low cytoplasmic Ca2+ concentration. Each of these mechanisms acts on a different timescale, providing layered control over signal duration and intensity.
<image>A signal amplification cascade diagram. At the top, one ligand binds one receptor. The receptor activates multiple G proteins (amplification x10). Each G protein activates one adenylyl cyclase, which produces many cAMP molecules (amplification x100). cAMP activates multiple PKA molecules (amplification x10). Each PKA phosphorylates multiple target enzymes (amplification x10). The total amplification from one signal to millions of product molecules is indicated at the bottom. Side annotations show where termination mechanisms act: phosphodiesterase degrades cAMP, phosphatases remove phosphate groups, GTPase inactivates G proteins.</image>
IX. Clinical Connections
Defects in signaling pathways underlie numerous diseases. Cholera toxin locks Gs-alpha in its GTP-bound active state, causing constitutive activation of adenylyl cyclase. The resulting flood of cAMP in intestinal epithelial cells drives massive chloride and water secretion, producing the severe, life-threatening diarrhea characteristic of cholera. Pertussis toxin (from the whooping cough bacterium) inactivates Gi-alpha, preventing it from inhibiting adenylyl cyclase and similarly disrupting cAMP regulation.
In cancer, mutations in signaling components transform normal growth-promoting pathways into oncogenic drivers. Approximately 30% of all human cancers harbor activating mutations in Ras, a small GTPase that relays signals from RTKs to the MAPK cascade. Mutant Ras is locked in its GTP-bound active form, continuously stimulating cell proliferation even in the absence of growth factor signals. Understanding these molecular defects has opened the door to targeted cancer therapies that specifically block the aberrant signaling events driving tumor growth.
