Cell signaling Study Guide

📖 Core Concepts Cell signaling – communication process where a first messenger (ligand) binds a receptor, triggering an intracellular signal that alters cell behavior. Ligand polarity determines receptor location: Hydrophilic/peptide → cannot cross membrane → membrane‑bound receptors. Lipid‑soluble (steroid, prostaglandin) → diffuse through membrane → intracellular receptors. Signaling distance categories Autocrine – acts on the same cell that released it. Intracrine – ligand binds receptors inside the same cell (cytoplasm/nucleus). Juxtacrine – requires direct cell‑cell contact (e.g., Notch). Paracrine – short‑range diffusion to nearby cells. Endocrine – long‑range transport via blood. Receptor families Ion‑channel‑linked – ligand opens/closes an ion‑selective pore. G‑protein‑coupled receptors (GPCRs) – 7‑TM proteins; act as guanine‑nucleotide‑exchange factors for heterotrimeric G proteins. Enzyme‑linked (catalytic) receptors – possess intrinsic kinase activity (e.g., RTKs) → autophosphorylation. Intracellular (nuclear) receptors – bind lipophilic ligands, translocate to nucleus, regulate transcription. Second messengers – small molecules (cAMP, Ca²⁺, IP₃, DAG) that amplify the original signal. One activated receptor can generate many messenger molecules. Signal attenuation – receptor endocytosis, phosphorylation, or adaptation reduces signaling intensity. --- 📌 Must Remember Ligand‑solubility rule: Hydrophilic → membrane receptor; Lipophilic → intracellular receptor. GPCR activation: ligand → receptor conformational change → GDP→GTP exchange on Gα → α‑subunit dissociates from βγ. Gα subunit outcomes Gαₛ → stimulates adenylyl cyclase → ↑cAMP. Gαᵢ/ₒ → inhibits adenylyl cyclase → ↓cAMP. Gαq/11 → activates phospholipase C → IP₃ + DAG → ↑Ca²⁺ + PKC activation. cAMP signaling: $cAMP = \text{adenylyl cyclase activity} \times \text{time}$; activates PKA → phosphorylates target proteins. MAPK/ERK cascade (simplified): EGFR autophosphorylation → GRB2 → SOS → RAS‑GTP → RAF → MEK → ERK → transcription factor phosphorylation. Ion‑channel receptor example: GABA\(A\) opens Cl⁻ channel → hyperpolarization → ↓ neuronal excitability. Clinical note: ≈ 1/3 of FDA‑approved drugs target GPCRs; RTK mutations are common oncogenic drivers. Exocytosis: bulk release of large, hydrophilic ligands (e.g., neurotransmitters, hormones) via vesicle‑plasma‑membrane fusion. --- 🔄 Key Processes GPCR → cAMP pathway Ligand binds GPCR → Gαₛ exchanges GDP for GTP → Gαₛ activates adenylyl cyclase → ↑cAMP → PKA activation → downstream phosphorylation. GPCR → IP₃/DAG pathway Ligand binds GPCR → Gαq activates phospholipase Cβ → PIP₂ → IP₃ + DAG → IP₃ releases Ca²⁺ from ER; DAG activates PKC. RTK (e.g., EGFR) → MAPK cascade Ligand (EGF) binds → receptor dimerizes & autophosphorylates → Grb2‑SOS recruitment → RAS‑GTP → RAF → MEK → ERK → nuclear transcription. Intracellular steroid receptor activation Lipid‑soluble hormone diffuses → binds cytoplasmic/nuclear receptor → conformational change → dimerization → DNA binding → transcriptional regulation. Exocytosis of peptide hormones Synthesis → packaging into secretory vesicle → Ca²⁺‑triggered vesicle fusion (porosome) → release into extracellular space. Receptor down‑regulation Activated receptor → phosphorylation → recruitment of adaptor proteins → clathrin‑mediated endocytosis → lysosomal degradation or recycling. --- 🔍 Key Comparisons Peptide vs. Lipid ligand Peptide: polar, cannot cross membrane → membrane‑bound receptor. Lipid: non‑polar, diffuses through membrane → intracellular receptor. Cell‑surface receptor families Ion‑channel – fast (ms), direct ion flux. GPCR – medium (seconds–minutes), second‑messenger amplification. Enzyme‑linked (RTK) – medium‑slow, phosphorylation cascades. Signaling distance Autocrine = same cell. Paracrine = nearby cells (diffusion). Juxtacrine = direct contact (membrane‑bound ligand). Endocrine = distant cells (bloodstream). Gα subunit functional outcome Gαₛ → ↑cAMP. Gαᵢ → ↓cAMP. Gαq → ↑Ca²⁺ + PKC. Second messenger vs. Direct ion flow Second messenger (cAMP, IP₃) → signal amplification, slower onset. Ion channel → immediate change in membrane potential. --- ⚠️ Common Misunderstandings “All peptide hormones bind intracellular receptors.” – False; peptides are hydrophilic and require membrane receptors. “GPCRs always increase cAMP.” – Only Gαₛ does; Gαᵢ/o decreases cAMP, Gαq triggers Ca²⁺. “Receptor internalization stops signaling.” – Internalized receptors can continue signaling from endosomes. “Autocrine = endocrine.” – Autocrine acts on the same cell; endocrine acts on distant cells via blood. “Second messengers are the same as the original ligand.” – They are distinct intracellular molecules that amplify the signal. --- 🧠 Mental Models / Intuition Lock‑and‑key + door model: Ligand = key; receptor = lock; the “door” (membrane) opens either an ion channel, a G‑protein switch, or a catalytic engine. Amplifier analogy: One activated GPCR = many cAMP molecules (like one microphone powering many speakers). Telephone network: GPCR (operator) hands the call to G proteins (switchboard), which route the message to downstream effectors (cAMP, Ca²⁺). Bistable switch (Xenopus oocyte): MAPK cascade can act like a light switch that stays “on” once a threshold is crossed. --- 🚩 Exceptions & Edge Cases Membrane‑bound steroid receptors exist (e.g., rapid estrogen signaling) → non‑genomic, fast responses. βγ subunits of G proteins can activate ion channels or PI3K independently of α subunit. Ligand‑induced receptor dimerization can occur for some GPCRs (biased agonism). Intracellular receptors can also mediate rapid, non‑transcriptional actions (e.g., membrane‑associated glucocorticoid receptors). Gaseous messengers (NO, CO, H₂S) diffuse freely and do not require traditional receptors. --- 📍 When to Use Which Fast synaptic transmission → Ion‑channel‑linked receptors (e.g., GABA\(A\), nicotinic ACh). Hormones that are peptides or cannot cross membrane → GPCRs or RTKs depending on downstream effect (second messenger vs. phosphorylation cascade). Growth factors, cytokines → Enzyme‑linked (RTK) receptors for MAPK/ERK pathways. Steroid, thyroid, retinoic acid hormones → Intracellular (nuclear) receptors for gene‑expression changes. Short‑range developmental cues → Paracrine signaling (FGF, Wnt, Hedgehog). Long‑range systemic regulation → Endocrine signaling (insulin, cortisol). --- 👀 Patterns to Recognize 7‑transmembrane segments → GPCR. Tyrosine‑phosphorylation motifs (YXXM) → RTK activation. Rapid Cl⁻ influx → GABA\(A\) or glycine receptor activation (hyperpolarization). cAMP rise + PKA phosphorylation → Gαₛ‑coupled GPCR stimulus. IP₃‑induced Ca²⁺ release + DAG‑PKC activation → Gαq‑coupled GPCR stimulus. Receptor autophosphorylation → Grb2‑SOS recruitment → MAPK cascade. Down‑regulation + ligand‑dependent internalization → high‑ligand exposure or prolonged stimulation. --- 🗂️ Exam Traps Confusing paracrine with juxtacrine – juxtacrine requires direct membrane contact (Notch), paracrine relies on diffusion. Assuming all GPCR drugs act on the same G protein – many are biased toward specific Gα subunits or βγ pathways. Mixing up Gαₛ vs. Gαᵢ effects on cAMP – opposite actions; a common distractor. Believing receptor internalization always terminates signaling – internalized RTKs can continue MAPK signaling from endosomes. Attributing rapid effects of steroid hormones to transcription – rapid “non‑genomic” actions occur via membrane‑associated receptors, not classic nuclear transcription. Mistaking ion‑channel receptors for GPCRs – ion‑channel receptors lack G‑protein coupling; they directly open a pore. ---