Cell signaling

1. Definition Signaling crosstalk refers to the interaction between two or more cellular signaling pathways such that activity in one pathway influences the activity, intensity, duration, localization, or outcome of another pathway. A signaling pathway therefore rarely functions as an isolated linear sequence. Cellular response = integration of multiple signaling inputs rather than activation of a single pathway. For example: Growth factor + cytokine + Ca²⁺ + metabolic signals may converge on common transcription factors and determine whether a cell: 2. Basic Concept A simplified linear pathway is: In reality: This interaction is crosstalk. 3. Why Crosstalk Is Important Crosstalk allows cells to: 1. IntegrateRead More →

1. Overview Cell signaling must solve two apparently opposite problems: Thus, effective signaling depends on a balance: Signal amplification → appropriate cellular response → signal termination → restoration of basal state This balance is essential for homeostasis, sensitivity, specificity, and prevention of pathological signaling. 2. Signal Amplification Definition Signal amplification is the process by which activation of a relatively small number of receptors produces a much larger number of downstream signaling events. A single ligand–receptor interaction can therefore result in: Many second-messenger molecules → many activated enzymes → many phosphorylated substrates 3. General Amplification Cascade This is one of the fundamental principles of signal transduction.Read More →

1. Overview Cyclic AMP (cAMP) is a major intracellular second messenger that converts extracellular signals into intracellular responses. The classical pathway is: Ligand → GPCR → G protein → adenylyl cyclase → cAMP → PKA → protein phosphorylation → cellular response The pathway is particularly important in: The key concept is that protein kinase cascades amplify and distribute the signal. 2. What Is cAMP? cAMP = cyclic adenosine 3′,5′-monophosphate It is synthesized from ATP by: Adenylyl cyclase (AC) The reaction is: ATP → cAMP + PPi cAMP is a diffusible intracellular second messenger. 3. Classical cAMP Signaling Pathway The pathway can be remembered as: GsRead More →

1. Overview Notch signaling is a highly conserved juxtacrine cell–cell communication pathway that regulates: Unlike many signaling pathways, Notch signaling generally does not require a freely diffusible ligand. Signaling occurs between adjacent cells through direct receptor–ligand interaction. Core concept Signal-sending cell → Notch ligand → Notch receptor on adjacent cell → proteolytic cleavage → NICD → nucleus → transcriptional regulation 2. Components of the Notch Pathway The canonical pathway contains four major components: Component Major examples Location Notch receptor NOTCH1–NOTCH4 Cell membrane Ligands Delta-like (DLL1, DLL3, DLL4), Jagged (JAG1, JAG2) Adjacent cell membrane γ-Secretase complex Presenilin, nicastrin, APH-1, PEN-2 Membrane Nuclear transcription machinery CSL/RBP-Jκ, MAML,Read More →

Master’s-Level Cell Biology & Advanced Molecular Biology Notes 1. Definition Hedgehog (Hh) signaling is a highly conserved developmental signaling pathway that regulates: In vertebrates, the best-known Hedgehog ligand is: Sonic Hedgehog (SHH) The canonical pathway can be summarized as: SHH → PTCH1 → SMO → GLI → nucleus → target-gene transcription A particularly important feature of vertebrate Hedgehog signaling is its close association with the: Primary cilium 2. The Core Pathway Core memory Hedgehog → PTCH → SMO → GLI → nucleus 3. Hedgehog Ligands The mammalian Hedgehog family contains three major ligands: Sonic Hedgehog — SHH Most extensively studied. Indian Hedgehog — IHH ImportantRead More →

Master’s-Level Cell Biology & Advanced Molecular Biology Notes 1. Definition Transforming growth factor-β (TGF-β) signaling is a major cell-signaling pathway that regulates: The classical pathway is: TGF-β → Type II receptor → Type I receptor → SMAD2/3 → SMAD4 → nucleus → transcription A key feature is that TGF-β receptors are serine/threonine kinase receptors, unlike receptor tyrosine kinases. 2. The Core Pathway Core memory TGF-β → TβRII → TβRI → SMAD2/3 → SMAD4 → nucleus 3. TGF-β Superfamily TGF-β signaling belongs to a much larger superfamily. Important members include: These ligands regulate development, tissue homeostasis and differentiation. 4. TGF-β Isoforms The three major mammalian TGF-βRead More →

Master’s-Level Cell Biology & Advanced Molecular Biology Notes 1. Definition The Wnt/β-catenin pathway is a major cell-signaling pathway involved in: The pathway is especially important because β-catenin functions both as a signaling molecule and as a structural component of adherens junctions. The canonical pathway can be summarized as: Wnt → Frizzled + LRP5/6 → Dishevelled → inhibition of β-catenin destruction complex → β-catenin accumulation → nucleus → TCF/LEF → gene transcription 2. The Central Concept The pathway essentially controls the stability of β-catenin. Without Wnt β-catenin is continuously targeted for degradation. With Wnt β-catenin degradation is inhibited. 3. What Is Wnt? Wnt proteins are secretedRead More →

Master’s-Level Cell Biology & Advanced Molecular Biology Notes 1. Definition The PI3K–AKT–mTOR pathway is a major intracellular signaling network that integrates extracellular signals, nutrient availability, cellular energy status and stress to regulate: The canonical pathway is: Growth factor → RTK → PI3K → PIP3 → AKT → mTORC1 → protein synthesis/growth It operates alongside pathways such as RAS–MAPK and frequently cross-talks with them. 2. Core Pathway Core memory PI3K → PIP3 → AKT → mTORC1 3. What Is PI3K? PI3K = Phosphoinositide 3-kinase PI3Ks are lipid kinases that phosphorylate membrane phosphoinositides. The most important reaction in the canonical pathway is: PIP2 → PIP3 More specifically:Read More →

Master’s-Level Cell Biology & Advanced Molecular Biology Notes 1. Definition The Ras–MAPK pathway is a major intracellular signaling cascade that converts extracellular signals, particularly growth-factor signals, into changes in cell behavior and gene expression. It is particularly important for: The canonical pathway is: RTK → GRB2 → SOS → RAS → RAF → MEK → ERK → nucleus → gene expression 2. The Core Pathway Core memory RAS → RAF → MEK → ERK This four-component sequence is the central MAPK module. 3. What Is RAS? RAS is a family of small monomeric GTPases. Important RAS proteins include: RAS acts as a molecular switch. InactiveRead More →