Cellular Communication and Signal Transduction Notes

Common Principles of Intracellular Signaling Pathways

• Structural and Functional Characteristics:

  • Signal transduction pathways feature a multi-layered, hierarchical structure.

  • Signal Amplification: Initial signals are amplified exponentially as an individual upstream signaling component activates multiple downstream signaling molecules.

  • Multi-Functional Divergence: A single initial signal can simultaneously regulate multiple cellular functions by branching into distinct intracellular cascades.

  • Control Mechanisms: Signaling intensity and duration are tightly controlled through both constitutive and regulated feedback mechanisms.

  • Desensitization: Continuous or repeated presence of a ligand often induces receptor desensitization, serving as a negative feedback mechanism that terminates signaling to prevent overstimulation.

• Case Example — Astrocyte Response to Local Brain Infection:

  • Scenario: During a local infection in the brain, a single cytokine ligand binds to its corresponding receptor (a receptor tyrosine kinase) on an astrocyte.

  • Robust Response via Amplification: High-magnitude cellular responses require upstream signaling components to sequentially activate numerous downstream components.

  • Concurrent Cellular Processes via Divergence: The activated cytokine receptor binds to multiple distinct adaptor protein types, each initiating an independent signaling cascade. This triggers parallel processes:

  • Morphological changes in the cell structure.

  • Secretion of proteins and additional cytokines into the extracellular space.

  • Cell proliferation to form a physical glial scar surrounding the infection site.

  • Controlled Proliferation via Negative Feedback: To prevent unchecked cellular division (which could lead to an astrocytoma tumor), negative feedback loops cease intracellular signaling despite the continued binding of extracellular cytokines.

Classification of Extracellular Signaling Mechanisms

• Modes of Extracellular Communication:

  • Synaptic Signaling: Highly localized transmission where neurotransmitters are released across a specialized synaptic cleft to act on an adjacent target membrane.

  • Contact-Mediated (Contact-Dependent) Signaling: Directly requires physical contact between membrane-bound or membrane-associated proteins expressed on two adjacent cells. These proteins function simultaneously as ligands and receptors, frequently initiating reciprocal intracellular signaling cascades in both interacting cells.

  • Paracrine Signaling: Secreted signaling molecules (referred to as local hormones) diffuse through the local extracellular fluid to act on nearby target cells in close physical proximity.

  • Autocrine Signaling: Secreted local hormones bind to surface receptors expressed on the exact same cell that secreted them, regulating its own activity.

  • Endocrine Signaling: Endocrine cells secrete hormones directly into the bloodstream, allowing systemic dissemination throughout the entire organism to target distant tissue sites.

• Detailed Molecular Example of Contact-Mediated Signaling — Ephrins and Eph Receptors:

  • Functional Role: Mediates contact-dependent cell-cell recognition, cell guidance, and neurite outgrowth.

  • Receptor/Ligand Structure:

  • EphA and EphB Receptors: Transmembrane proteins featuring extracellular Ephrin-binding domains, Cys-rich regions, Fibronectin type III (FNIII) repeats, a transmembrane (TM) domain, an intracellular tyrosine kinase (TK) domain, a SAM domain, and a C-terminal PDZ-binding motif.

  • Ephrin-A Ligands: Anchored to the outer cell membrane via a glycosylphosphatidylinositol (GPI) anchor.

  • Ephrin-B Ligands: Transmembrane proteins containing an intracellular domain with a PDZ-binding motif.

  • Forward and Reverse Intracellular Signaling Pathways:

  • Ephrin-A Signaling (Reverse): Transduces signals through FYN kinase (regulating integrin-dependent adhesion) and Erk.

  • Ephrin-B Signaling (Reverse): Transduces signals through MMP8, JAK/STAT3 (promoting invasion), Src, and GRB4/RAC1 (promoting epithelial-mesenchymal transition, EMT).

  • EphA Receptor Signaling (Forward): Activates Src, Focal Adhesion Kinase (FAK) $\rightarrow$ RHOA (cell migration), JAK2 $\rightarrow$ STAT3 (invasion), and PI3K $\rightarrow$ Akt (cell proliferation).

  • EphB Receptor Signaling (Forward): Activates RAC1 (EMT), CDC42, and RHOA (cell migration).

Receptor Classes and Activation Mechanisms

• Ionotropic Receptors:

  • Definition: Ligand-gated ion channels.

  • Structural Architecture: Multi-subunit protein complexes arranged around a central ion-conducting pore.

  • Mechanism of Action: Ligand binding to the extracellular domain induces a conformational change that alters the open-state probability of the channel pore.

  • Ion Selectivity: Selectively permeable to specific ions, predominantly $\text{Na}^+/\text{K}^+$, $\text{Na}^+/\text{K}^+/\text{Ca}^{2+}$, or $\text{Cl}^-$.

  • Function: Primary receptor type mediating fast chemical neurotransmission at neuronal synapses.

• Metabotropic Receptors:

  • Definition: G protein-coupled receptors (GPCRs).

  • Structural Architecture: Monomeric receptor proteins containing $7$ hydrophobic transmembrane $\alpha$-helical domains.

  • Mechanism of Action: Extracellular ligand binding stabilizes an active receptor conformation, promoting physical interaction with and activation of an intracellular heterotrimeric G protein, initiating downstream signaling cascades.

• Enzyme-Linked Receptors:

  • Structural Architecture: Transmembrane receptor proteins possessing an extracellular ligand-binding domain and an intracellular domain with intrinsic enzymatic activity (or directly associated with an enzyme subunit).

  • Enzymatic Categories: Includes receptor tyrosine kinases (RTKs), serine/threonine kinases, tyrosine phosphatases, serine/threonine phosphatases, and guanylyl cyclases.

  • Enzymatic Reactions:

  • Tyrosine Kinases: $\text{Protein-Tyr-OH} + \text{ATP} \rightarrow \text{Protein-Tyr-O-PO}_3^{2-} + \text{ADP}$

  • Tyrosine Phosphatases: $\text{Protein-Tyr-O-PO}_3^{2-} + \text{H}_2\text{O} \rightarrow \text{Protein-Tyr-OH} + \text{P}_i$

  • Serine/Threonine Kinases: $\text{Protein-Ser/Thr-OH} + \text{ATP} \rightarrow \text{Protein-Ser/Thr-O-PO}_3^{2-} + \text{ADP}$

  • Serine/Threonine Phosphatases: $\text{Protein-Ser/Thr-O-PO}_3^{2-} + \text{H}_2\text{O} \rightarrow \text{Protein-Ser/Thr-OH} + \text{P}_i$

  • Sequence of Events for Receptor Tyrosine Kinases (e.g., TrkA / NGF Receptor):

  1. Dimeric ligand (e.g., NGF dimer) binds to the extracellular domains of two receptor monomers.

  2. Receptors undergo ligand-induced dimerization.

  3. Intracellular kinase domains cross-phosphorylate tyrosine residues on the opposing receptor tail (trans-autophosphorylation).

  4. Phosphorylated tyrosine residues function as specific docking sites for intracellular effector proteins.

  5. Docked effector proteins trigger downstream pathways (e.g., via small monomeric G proteins).

• Intracellular and Nuclear Receptors:

  • Mechanism of Action: Target receptors located inside the cytoplasm or nucleus, activated by lipophilic signaling molecules that freely cross the plasma membrane lipid bilayer via passive diffusion.

  • Primary Function: Act as direct transcriptional modulators by binding specific gene promoter sequences to activate or repress transcription of target genes.

  • Examples: Glucocorticoid receptors, thyroid hormone receptors.

G Proteins and Their Downstream Targets

• Common Principles of G Proteins:

  • Definition: Specialized proteins defined by their capacity to bind and hydrolyze guanosine triphosphate (GTP).

  • Core Function: Act as molecular switches coupling receptor activation at the cell surface to downstream effector enzyme/channel regulation.

  • Conformational States: Active when bound to GTP; inactive when bound to GDP.

  • Hydrolysis Reaction: $\text{GTP} + \text{H}_2\text{O} \xrightarrow{\text{GTPase}} \text{GDP} + \text{H}_2\text{PO}_4^-$

• Classification of G Proteins:

  • Heterotrimeric G Proteins: Multi-protein complexes comprising three distinct structural subunits ($\alpha$, $\beta$, $\gamma$); activated primarily by GPCRs.

  • Small Monomeric G Proteins: Single polypeptide chains; activated downstream of RTKs and other non-GPCR mechanisms.

• Cycle of Heterotrimeric G Protein Activation and Inactivation:

  1. Basal State: Inactive heterotrimer ($\alpha\beta\gamma$) is bound to GDP on the $\alpha$ subunit.

  2. Activation: Ligand binding to GPCR induces a conformational change that promotes interaction with the G protein, driving the exchange of bound GDP for GTP on the $\alpha$ subunit.

  3. Dissociation: GTP binding triggers $\text{G}_\alpha$ activation, causing it to dissociate from both the GPCR and the $\text{G}_{\beta\gamma}$ subunit complex.

  4. Effector Engagement: Both liberated $\text{G}_\alpha\text{-GTP}$ and $\text{G}_{\beta\gamma}$ complexes independently interact with specific target effector proteins.

  5. Hydrolysis & Inactivation: A GTPase Activating Protein (GAP) or intrinsic $\text{G}_\alpha$ GTPase activity hydrolyzes bound GTP to GDP, releasing inorganic phosphate ($\text{P}_i$).

  6. Reassociation: Inactive $\text{G}_\alpha\text{-GDP}$ reassociates with the $\text{G}_{\beta\gamma}$ complex to restore the basal heterotrimeric complex.

• Primary Heterotrimeric G Protein Families and Effector Targets:

  • $\text{G}_s$ Family:

  • Effector Target: Activates adenylyl cyclase.

  • Second Messenger Effect: Increases intracellular cyclic AMP ($\text{cAMP} \uparrow$).

  • $\text{G}_i$ Family:

  • Effector Target: Inhibits adenylyl cyclase.

  • Second Messenger Effect: Decreases intracellular cyclic AMP ($\text{cAMP} \downarrow$).

  • $\text{G}_t$ (Transducin) Family:

  • Effector Target: Activates cGMP phosphodiesterase.

  • Second Messenger Effect: Decreases intracellular cyclic GMP ($\text{cGMP} \downarrow$).

  • $\text{G}_q$ Family:

  • Effector Target: Activates phospholipase C (PLC).

  • Second Messenger Effect: Increases diacylglycerol ($\text{DAG} \uparrow$) and inositol trisphosphate ($\text{IP}_3 \uparrow$).

• Receptor-G Protein Coupling Specificity:

  • Selective Activation: Each specific metabotropic receptor subtype couples strictly to a single class of heterotrimeric G protein ($\text{G}_s$, $\text{G}_i$, $\text{G}_q$, or $\text{G}_t$).

  • Tissue-Specific Diversity Example (Adrenergic Receptors):

  • Ligands: Adrenaline (Epinephrine) and Noradrenaline (Norepinephrine).

  • $\alpha_1$ Adrenergic Receptor: Couples to $\text{G}_{q/11}$ $\rightarrow$ Activates Phospholipase C $\rightarrow$ Cleaves $\text{PIP}_2$ to $\text{IP}_3$ and DAG $\rightarrow$ Increases cytosolic $\text{Ca}^{2+}$ $\rightarrow$ Mediates smooth muscle contraction.

  • $\alpha_2$ Adrenergic Receptor: Couples to $\text{G}_i$ $\rightarrow$ Inhibits Adenylyl Cyclase (lowers cAMP) and blocks $\text{Ca}^{2+}$ influx $\rightarrow$ Mediates inhibition of neurotransmitter release and smooth muscle contraction.

  • $\beta$ Adrenergic Receptor: Couples to $\text{G}_s$ $\rightarrow$ Activates Adenylyl Cyclase (converts ATP to cAMP) $\rightarrow$ Mediates heart muscle contraction, smooth muscle relaxation, and glycogenolysis.

Small Monomeric G Proteins and the MAP Kinase Pathway

• Regulation of Monomeric G Protein Activation:

  • Guanine Nucleotide Exchange Factors (GEFs): Facilitate the release of GDP and binding of GTP, transitioning the monomeric G protein into its active state. GEFs are recruited and activated upstream by receptor tyrosine kinases (via adaptor proteins) and other signals.

  • GTPase Activating Proteins (GAPs): Accelerate the intrinsic GTPase hydrolysis activity of the monomeric G protein, converting bound GTP to GDP and driving rapid inactivation.

• Monomeric G Protein Families:

  • Ras Family:

  • Physiological Functions: Controls cell proliferation, cell differentiation, and cell survival.

  • Effector / Downstream Pathway: MAP kinase (MAPK) signaling pathway.

  • Rho Family:

  • Physiological Functions: Regulates actin cytoskeleton dynamics, cell morphology, and cell motility.

  • Effector / Downstream Pathway: ROCK kinase.

  • Rab Family:

  • Physiological Functions: Directs intracellular membrane trafficking, vesicle budding, targeting, and fusion.

  • Effector / Downstream Pathway: Various specialized trafficking effectors.

• Step-by-Step Sequence of the MAP Kinase (MAPK) Signaling Cascade:

  1. Extracellular signaling molecule binds and activates an RTK.

  2. RTK autophosphorylation creates a binding site for the adaptor protein GRB2.

  3. GRB2 recruits a GEF (Sos) to the membrane.

  4. GEF stimulates GDP/GTP exchange on Ras, activating it ($\text{Ras-GTP}$).

  5. Active $\text{Ras-GTP}$ binds and activates MAPKKK (Raf kinase).

  6. MAPKKK phosphorylates MAPKK (MEK) using ATP ($\text{ATP} \rightarrow \text{ADP}$), activating it.

  7. Active MAPKK phosphorylates MAPK (ERK) using ATP ($\text{ATP} \rightarrow \text{ADP}$), activating it.

  8. Active MAPK phosphorylates and regulates various target effector proteins and nuclear transcription factors.

• Pathology and Feedback Control of Ras Signaling:

  • Proto-Oncogene Role: Mutated versions of ras genes are present in $20\%\text{ to }30\%$ of all human tumors.

  • Oncogenic Mechanism: Specific point mutations (e.g., residues G12, G13, Q61) inhibit the GTPase capacity of Ras, rendering it incapable of hydrolyzing GTP even in the presence of GAPs. This results in chronically GTP-bound, constitutively active Ras ($\text{Ras}^*$), driving persistent MAP kinase signaling and uncontrolled cell proliferation.

  • Negative Feedback Control: In normal cells, Ras activity is tightly dampened by negative feedback. For example, during reactive astrocytosis, Neurofibromin 1 (NF1, a specific GAP protein) is expressed to hydrolyze $\text{Ras-GTP}$ to $\text{Ras-GDP}$, preventing progression to astrocytoma.

Second Messenger Systems

• Cyclic AMP (cAMP):

  • Synthesis: Produced from ATP by adenylyl cyclase, an enzyme activated by $\text{G}_s$ and inhibited by $\text{G}_i$ subunits.

  • Downstream Targets:

  • Protein Kinase A (PKA): A serine/threonine kinase that phosphorylates diverse proteins to modulate metabolic processes, ion membrane transport, and gene expression.

  • Cyclic Nucleotide-Gated Channels: Direct binding of cAMP modulates channel gating and ion permeability.

  • Degradation: Hydrolyzed to 5'-AMP by cAMP phosphodiesterases.

  • Pharmacological Relevance: Naturally occurring cAMP phosphodiesterase inhibitors (e.g., theophylline in tea and cocoa) act as bronchodilators. Isoform-specific inhibitors, such as PDE4 inhibitors, are used to treat chronic obstructive pulmonary disease (COPD).

• Cyclic GMP (cGMP):

  • Synthesis: Produced from GTP by guanylyl cyclase.

  • Downstream Targets:

  • Protein Kinase G (PKG): A serine/threonine kinase regulating cardiovascular and neuronal targets.

  • Cyclic Nucleotide-Gated Channels: Regulates ion influx (plays a central role in visual phototransduction in photoreceptors).

  • Degradation: Hydrolyzed to 5'-GMP by transducin-activated cGMP phosphodiesterase.

• Inositol Trisphosphate ($\text{IP}_3$) and Diacylglycerol (DAG):

  • Precursor: Phosphatidylinositol bisphosphate ($\text{PIP}_2$), a specialized phospholipid localized in the inner leaflet of the plasma membrane.

  • Generation: Cleaved by Phospholipase C (PLC) into two distinct second messengers:

  • Diacylglycerol (DAG): Lipophilic cleavage product that remains embedded within the plasma membrane.

  • Inositol Trisphosphate ($\text{IP}_3$): Water-soluble cleavage product that diffuses rapidly through the cytosol.

  • PLC Isoforms: Major isoform PLC-$\beta$ is activated by $\text{G}_q$ proteins; other PLC isoforms are activated by receptor tyrosine kinases or elevated cytosolic $\text{Ca}^{2+}$.

  • Target Effector Interactions:

  • $\text{IP}_3$: Binds to and opens $\text{IP}_3$ receptors (ligand-gated $\text{Ca}^{2+}$ channels) on the endoplasmic reticulum (ER) membrane, releasing sequestered $\text{Ca}^{2+}$ into the cytosol.

  • DAG: Acts synergistically with released cytosolic $\text{Ca}^{2+}$ to bind and activate Protein Kinase C (PKC), a serine/threonine kinase that phosphorylates structural proteins, ion channels, and metabolic enzymes.

• Calcium Ions ($\text{Ca}^{2+}$):

  • Basal Cytosolic Concentration: Maintained extremely low in resting cells ($\le 100\,\text{nM}$).

  • Mechanisms Limiting Free Cytosolic $\text{Ca}^{2+}$:

  • Plasma Membrane $\text{Ca}^{2+}$ ATPase (PMCA) pump (extrudes $\text{Ca}^{2+}$ coupled with $\text{H}^+$).

  • $\text{Na}^+/\text{Ca}^{2+}$ exchanger (NCX) in the plasma membrane.

  • Sarco/Endoplasmic Reticulum $\text{Ca}^{2+}$ ATPase (SERCA) pump (actively sequesters cytosolic $\text{Ca}^{2+}$ into the ER lumen).

  • Cytosolic $\text{Ca}^{2+}$-binding buffer proteins.

  • Mechanisms Increasing Free Cytosolic $\text{Ca}^{2+}$ (Influx):

  • Plasma Membrane Channels: Voltage-gated $\text{Ca}^{2+}$ channels and ligand-gated $\text{Ca}^{2+}$ channels.

  • ER Membrane Channels: $\text{IP}_3$ receptors and Ryanodine receptors.

  • Effector Activation:

  • Free cytosolic $\text{Ca}^{2+}$ binds to the universal sensor protein Calmodulin (CaM).

  • $\text{Ca}^{2+}/\text{CaM}$ complex binds and activates downstream effectors, notably $\text{Ca}^{2+}/\text{calmodulin-dependent kinase type II (CaMKII)}$ (a serine/threonine kinase regulating ion channels, metabolic enzymes, and transcription) and Protein Phosphatase 2B (PP2B / Calcineurin).

Signal Transduction and Regulation of Gene Expression

• Principles of Signal-Mediated Gene Regulation:

  • Functional Outcome: Signal transduction cascades alter gene transcription to regulate the synthesis of new mRNA and structural/functional proteins.

  • Temporal Profile: Slow in onset ($>30\,\text{min}$ or $30\,\text{min} - 1\,\text{hour}$) but provides long-lasting functional cellular changes (lasting days or weeks) compared to fast, transient post-translational modifications like direct phosphorylation.

  • Molecular Mechanism: Intracellular signaling cascades activate specific transcriptional activator proteins, which translocate to the nucleus and bind Upstream Activating Sequences (UAS) or specific response elements near target gene promoters. This binding recruits general transcription factors and RNA polymerase to execute mRNA transcription.

• CREB (cAMP-Responsive Element Binding Protein) Signaling:

  • Activation Requirement: Requires phosphorylation at specific amino acid residues to gain transcriptional activity.

  • Activating Kinases: Phosphorylated by PKA, MAPK, and CaMKII downstream of distinct receptor inputs.

  • Target Promoter Action: Active phosphorylated CREB binds cAMP-responsive elements (CRE) in target gene promoters to induce expression of critical neural genes, including BDNF, neuropeptides, tyrosine hydroxylase, and c-fos.

• c-fos Signaling and Immediate Early Genes:

  • Baseline State: Present at minimal levels in unstimulated, resting cells.

  • Classification: Designated as an "immediate early gene" because its transcription is rapidly activated by pre-existing transcription factors (CREB phosphorylated by PKA, MAPK, or CaMKII) without requiring any prior protein synthesis.

  • Transcriptional Cascade Logic: Newly synthesized c-fos protein functions as a potent transcriptional activator. It dimerizes with c-jun to form the AP-1 complex, binding TPA Response Elements (TRE) to stimulate transcription of numerous delayed target genes, amplifying and diversifying the original signal.

Examples of Neuronal Signal Transduction

• Regulation of Catecholamine Synthesis — Model of Convergent Signaling:

  • Catecholamine Neurotransmitters: Dopamine, Norepinephrine (Noradrenaline), and Epinephrine (Adrenaline).

  • Rate-Limiting Step: Catalyzed by the enzyme Tyrosine Hydroxylase (TH).

  • Post-Translational Regulation: Direct phosphorylation of TH enhances its catalytic activity, increasing catecholamine synthesis.

  • Convergent Molecular Mechanism: Multiple independent signaling pathways converge to regulate TH:

  • Direct protein phosphorylation of TH is mediated by PKA, CaMKII, MAPK, and PKC.

  • Long-term gene transcription of TH is regulated by phosphorylated CREB.

• Neurotrophin Signaling — Model of Divergent Signaling:

  • Ligand: Nerve Growth Factor (NGF).

  • Primary Receptor: TrkA (a receptor tyrosine kinase expressed on sensory and sympathetic neurons required for cell survival, differentiation, and neurite outgrowth).

  • Divergent Molecular Mechanism: Binding of a single NGF ligand to TrkA simultaneously triggers three independent intracellular signaling cascades:

  1. Phospholipase C (PLC) Pathway: Generates $\text{IP}_3$ and DAG $\rightarrow$ Drives cytosolic $\text{Ca}^{2+}$ release and PKC activation $\rightarrow$ Promotes neurite outgrowth and differentiation.

  2. Ras / GEF Pathway: Activates Ras $\rightarrow$ MAP kinase cascade $\rightarrow$ Promotes neurite outgrowth and differentiation.

  3. PI3 Kinase Pathway: Recruits adaptor proteins $\rightarrow$ Activates PI3 kinase $\rightarrow$ Activates Akt kinase $\rightarrow$ Promotes cell survival.

Questions, Answers, and Self-Assessment Reviews

• Student Questions and Answers:

  • Question: Explanation of contact-mediated signaling.

  • Answer: Contact-mediated signaling involves membrane-bound or membrane-associated proteins on two neighboring cells physically touching. One or both of these membrane proteins initiate intracellular signaling cascades in the respective cells (e.g., Ephrin ligands and Eph receptors in axonal pathfinding).

  • Question: Overview of the MAP kinase (MAPK) signal transduction pathway.

  • Answer: Ligand binding to a receptor tyrosine kinase triggers autophosphorylation, recruiting GRB2 and GEF to activate Ras ($\text{Ras-GDP} \rightarrow \text{Ras-GTP}$). Active Ras sequentially activates MAPKKK (Raf), which phosphorylates MAPKK (MEK), which phosphorylates MAPK (ERK). Active MAPK then phosphorylates downstream target enzymes and transcription factors.

• Structural Comparison — Heterotrimeric vs. Monomeric G Proteins:

  • Similarities:

  1. Both are functionally active when bound to GTP and inactive when bound to GDP.

  2. Inactivation/GTP hydrolysis in both is accelerated by GTPase Activating Proteins (GAPs).

  • Differences:

  1. Activating Mechanism: Heterotrimeric G proteins are directly activated by GPCRs, whereas monomeric G proteins are activated by GEFs.

  2. Subunit Interaction and Multiplicity: Activated heterotrimeric G proteins dissociate into two active signaling units ($\text{G}_\alpha\text{-GTP}$ and $\text{G}_{\beta\gamma}$) that can simultaneously regulate multiple effector types, whereas monomeric G proteins act as single subunits targeting a single direct downstream effector pathway.

• Self-Assessment Statements & Responses:

  • Signal transduction pathways have a multi-layered, hierarchical structure. True.

  • Contact-mediated signaling involves proteins serving as ligands as well as receptors on the membrane of two neighbouring cells. It often elicits signalling in both cells. True.

  • Receptor desensitization is a negative feedback mechanism preventing uncontrolled, sustained signalling. True.

  • During paracrine signaling, the secreted ligand enters the bloodstream and is disseminated throughout the body. False. (Paracrine signaling involves local interstitial diffusion acting on nearby target cells; systemic blood dissemination defines endocrine signaling).

  • Ligands of ionotropic receptors initiate signaling by eliciting the opening of a channel pore, leading to diffusion of ions across membrane.

  • Metabotropic receptors elicit cellular changes by activating G proteins.

  • Receptor tyrosine kinases belong to the class of enzyme-linked receptors.

  • Intracellular receptors are activated by lipophilic compounds and often function to modulate gene transcription.

  • The specificity of signaling through metabotropic receptors is due to each class of metabotropic receptors only binding one kind of heterotrimeric G protein.

  • Heterotrimeric G proteins are activated by metabotropic receptors.

  • Fast inactivation of heterotrimeric G proteins requires a GTPase activating protein (GAP).

  • In their GDP-bound state, heterotrimeric G proteins bind metabotropic receptors.

  • $\text{G}_s$ proteins activate an enzyme called adenylyl cyclase, whereas $\text{G}_i$ proteins inhibit an enzyme called adenylyl cyclase.

  • Protein kinase A is activated by cAMP.

  • Diacylglycerol (DAG) and inositol trisphosphate ($\text{IP}_3$) are generated by phospholipase C.

  • cGMP can modulate the membrane potential through cyclic nucleotide-gated channels.

  • $\text{IP}_3$ activates $\text{IP}_3$ receptors located in the membrane of the ER.

  • To activate kinases, calcium ions first need to bind to calmodulin.

  • Cellular changes elicited by alterations in gene expression can be observed after 30 min – 1 hour (slow in onset!).

  • Transcriptional activators bind to DNA near the promoter of a target gene to effect changes in transcription.

  • The transcription factor cAMP-responsive element binding protein (CREB) is activated by protein phosphorylation involving protein kinase A.

  • c-fos is a transcriptional activator whose own expression is tightly regulated.

  • Activation of an individual signaling pathway often leads to activation of multiple different target proteins if the signaling is divergent (e.g., the signaling of NGF via the TrkA receptor).

  • A cell process that has to be regulated by multiple different extracellular signals often requires a convergent signaling pathway (e.g., the regulation of catecholamine synthesis).

  • Processes that require transient regulation (short-term change in the order of hours) are often regulated via protein phosphorylation, whereas long-term changes (lasting days) require additional transcriptional regulation.