Case 1 - BBS2042

Cell Communication Routes

1. Learning Goals

  • Contact dependent signalling (adherence, tight, gap junctions)

  • Autocrine, paracrine, and intracrine signalling

  • Long distance signalling pathways (endocrine)

  • Explanation of the following pathways:

    • Notch-Delta (juxtasacrine)

    • WNT (short distance)

    • TGF-beta (short distance)

    • Hedgehog (short distance)

    • JAK-STAT (short distance)

Distance the signal travels

  • Intracrine: Signals act inside the same cell.

  • Autocrine: Signals act on the same cell. The cell secreted it and then re-binds it.

  • Paracrine: Signals affect nearby cells.

  • Endocrine: Signals act on distant cells via blood.

  • Juxtacrine: Direct contact between cells (e.g., Notch, junctions).

2. Contact-Dependent (Juxtacrine) Signalling

  • Definition: Contact-dependent signalling requires direct physical contact between neighbouring cells, unlike paracrine or endocrine signalling, which utilize soluble ligands diffusing through the extracellular space.

  • Types of Contact-Dependent Signalling:

    1. Junctional signalling (e.g., adherens, tight, gap junctions)

    2. Receptor-ligand juxtacrine signalling (e.g., Notch-Delt)

    3. Cell-ECM contact signalling (involving integrins; not strictly cell-to-cell, but often discussed together).

3. Adherens Junctions (AJs)

Core Function

  • Mechanical Adhesion: AJs provide mechanical adhesion between cells.

  • Force Transmission: They participate in the transmission of forces across tissues.

  • Signal Transduction: Signal transduction related to cytoskeletal tension and gene expression control.

3.1 Molecular Architecture

  • Transmembrane Adhesion Molecules:

    • Classical Cadherins:

    • E-cadherin (found in epithelia)

    • N-cadherin (found in neuronal and mesenchymal cells)

    • VE-cadherin (in endothelial tissue)

    • P-cadherin (in placenta)

    • Key Features:

    • Homophilic Binding: E-cadherin binds to other E-cadherins.

    • Calcium Dependent: Adhesion is dependent on Ca²⁺ in the extracellular domains.

    • Strength of Adhesion: Increases with clustering of cadherins.

3.2 Cytoplasmic Adaptor Proteins

  • Roles of Cadherins Connect to Actin Cytoskeleton:

    • β-catenin: Links cadherin to α-catenin; acts as a transcriptional co-activator.

    • α-catenin: Connects junctions to actin filaments.

    • p120-catenin: Stabilizes cadherins at the membrane.

    • Vinculin: Recruited under tension to strengthen junctions.

3.3 Signalling Roles

  • Mechanosensing: AJs act as mechanochemical signalling hubs and sense:

    • Cell-to-cell contact

    • Mechanical force (tension and shear stress)

    • Cell density

    • Cytoskeletal dynamics

  • Core Signalling Mechanisms:

    • β-catenin Role:

    • In the membrane: β-catenin stabilizes cadherin adhesion.

    • In the nucleus: β-catenin activates Wnt target genes.

  • Junction-Signalling Balance:

    • Strong adhesion sequesters β-catenin.

    • Junction disassembly leads to β-catenin being released, escaping degradation and entering the nucleus, activating TCF/LEF.

4. Tight Junctions (TJs)

Core Function

  • Paracellular Barrier: TJs seal the paracellular space between cells.

  • Cell Polarity: They establish and maintain cell polarity.

  • Selective Permeability: They regulate the selective permeability of ions and molecules between cells.

4.1 Molecular Architecture

  • Transmembrane Proteins:

    • Claudins (≈27 types): Determine ion and solute selectivity.

    • Occludin: Maintains junction stability and regulation.

    • Junctional Adhesion Molecules (JAMs): Facilitate cell-to-cell adhesion and immune cell transmigration.

  • Cytoplasmic Scaffold Proteins: ZO-1, ZO-2, ZO-3 link junctions to actin and organize signalling complexes.

4.2 Signalling Roles

  • Active Functionality: TJs are involved actively in signalling rather than being passive seals.

  • Cellualr Sensing: They can sense:

    • Cell polarity

    • Barrier integrity

    • Mechanical stress

    • Inflammatory cues

  • Signalling Platforms:

    • ZO-1: Binds transcription regulators (e.g., ZONAB).

    • Changes in TJ integrity can influence cell proliferation and differentiation.

    • Crosstalk with Hippo, MAPK and RhoA pathways.

  • Functional Diversity: Distinct types of tight epithelia with various claudin combinations influence permeability.

4.3 Tight Junctions and Polarity Complexes

  • TJs anchor complexes such as:

    • PAR complex (Par3–Par6–aPKC)

    • Crumbs complex

    • Scribble complex

  • These determine apical membrane identity, directional signalling, and oriented cell division.

5. Gap Junctions

Core Function

  • Direct Cytoplasmic Communication: Enable direct communication between cells through intercellular channels without using extracellular signalling.

  • Coupling Mechanisms: Facilitate electrical and metabolic coupling.

5.1 Molecular Architecture

  • Connexins: About 21 human connexins formed into connexons (hemichannels).

    • Structure: 6 connexins form a connexon; two connexons (from adjacent cells) create a gap junction channel.

5.2 What Passes Through Gap Junctions?

  • Small Molecules:

    • Ions (Ca²⁺, K⁺)

    • cAMP, IP₃

    • Glucose, ATP (in certain tissues)

    • Electrical signals (especially in cardiac muscle)

  • Regulation: The functioning of gap junctions can be dynamically regulated by:

    • Connexin phosphorylation

    • Intracellular Ca²⁺ levels

    • pH changes

    • Voltage differences.

5.3 Signalling Roles

  • Transmission of Signals:

    • Do not activate receptors; instead, they facilitate communications of signals, including electrical signals, second messengers, and metabolic intermediates.

5.4 Connexins as Signalling Proteins

  • Connexins not only act as channels but also:

    • Interact with kinases (e.g., Src, PKC)

    • Bind cytoskeletal proteins

    • Influence gene expression indirectly; for instance, Cx43 can regulate the cell cycle without channel function.

6. Other Junctions

6.1 Desmosomes

  • Specialized Adhesive Junctions: Bind neighbouring cells and provide significant mechanical strength, especially in the epidermis and cardiac muscle.

  • Structure:

    • Cadherin proteins (desmogleins and desmocollins) connect through a plaque to intermediate filaments via:

    • Desmoplakin

    • Plakoglobin

    • Plakophillins

  • Functionality: Provide tissue integrity, resistance to mechanical stress, and play roles in signalling.

6.2 Hemidesmosomes

  • Specialized Junctions: Anchor epithelial cells to the basement membrane with high tensile strength and structural integrity.

  • Structure:

    • Cytoplasmic plaque binds to keratin filaments containing:

    • Plectin

    • BP230

    • Transmembrane complex binds to extracellular laminin-332 with

    • Integrin

    • Alpha-6-Beta-4

    • BP180

    • CD151

  • Types:

    • Type I: Stratified epithelia like skin (BP230 and BP180 included)

    • Type II: Simple epithelia lacking BP230 and BP180, relying primarily on plectin and integrin.

6.3 Focal Adhesions

  • Actin-Linked Cell-Matrix Junctions: Anchor cells to the extracellular matrix through transmembrane integrins.

  • Structure:

    • Integrins binding ECM molecules

    • Bundles of actin filaments in the cytosol

    • Linking proteins (like talin, vinculin, alpha-actinin) connecting integrins to actin.

7. Notch-Delta as Contact-Dependent Signalling

  • Instructive Function: Different from adherens, tight or gap junctions; it uses membrane-bound ligands and receptors to provide a one-way instructive signal, culminating with direct transcriptional control.

    • Purpose: Determines the fate of neighbouring cells that are in contact.

    • Example: Cell fate decisions and pattern formation.

7.1 Core Components

  • Notch Receptor:

    • A large single-pass transmembrane protein with variants Notch1-4 in humans.

    • Structure:

    • Extracellular domain with multiple EGF-like repeats

    • Negative regulatory region (NRR) to prevent activation

    • Transmembrane domain

    • Intracellular domain (NICD) acting as a transcriptional regulator.

  • Ligands:

    • Delta-like (DLL - including DLL1, DLL3, DLL4)

    • Jagged (JAG1, JAG2)

    • Key Point: Both receptor and ligand are membrane-anchored, requiring direct contact for signalling.

7.2 Mechanical Nature of Notch Activation

  • Mechanotransduction Dependency: Notch signalling seems solely dependent on the mechanical aspect of cell interactions.

  • Process of Activation:

    1. Cell-Cell Contact: Cell A expresses Notch, while cell B expresses Delta/Jagged.

    2. Ligand Binding: The ligand binds to the extracellular domain of Notch.

    3. Endocytosis-generated Pulling Force: The ligand-expressing cell undergoes endocytosis of the Notch-ligand complex, creating a mechanical force that:

    • Unfolds the NRR of Notch, exposing a cleavage site for protease action.

7.3 Proteolytic Cleavages: Turning Contact into a Signal

  • Following NRR opening:

    • S2 Cleavage:

    • Enzyme: ADAM metalloprotease

    • Occurs extracellularly, removing most of Notch ectodomain.

    • S3 Cleavage:

    • Enzyme: γ-secretase

    • Cleavage occurs within the transmembrane domain releasing NICD.

    • Key Point: The receptor becomes the signalling molecule itself.

7.4 Nuclear Signalling and Transcriptional Control

  • NICD Translocation: After entering the nucleus, NICD binds with:

    • CSL (CBF1/RBPJ in mammals)

    • Mastermind-like (MAML) co-activators

  • Function of the Complex: Converts CSL from a repressor to an activator, driving transcription of Notch target genes:

    • Key Target Genes: HES (transcriptional repressors), HEY (inhibit differentiation), NRARP (feedback regulation).

    • Function: Typically suppress differentiation and maintain progenitor states, enforcing binary fate decisions.

7.5 Lateral Inhibition: The Classic Notch Logic

  • Definition of Lateral Inhibition: A mechanism where one cell adopts a certain fate, prompting neighbouring cells to adopt a different fate.

  • Mechanism:

    1. Delta Expression Variability: Small stochastic differences in Delta expression arise.

    2. The cell with slightly higher Delta activates Notch in neighbouring cells.

    3. Activated Notch suppresses Delta expression and prevents differentiation in neighbours.

  • Outcome:

    • “Sender” cell differentiates while “Receiver” cells maintain progenitor identity.

  • Examples: Neurogenesis, sensory organ formation, and epidermal patterning.

8. Intracrine Signalling

  • Definition: Intracrine signalling occurs when a signalling molecule acts within the cell that produced it, which can involve molecules that are not secreted or signal before or without binding to a receptor at the membrane.

  • Key Features:

    • Characterized by speed, specificity, and no signal dilution.

8.1 Mechanisms

  • Types of Interactions:

    • Cytosolic or Nuclear Ligand-Receptor Interaction:

    • Ligand is produced intracellularly, and receptors are cytosolic or in organelle membranes.

    • Receptor-Independent Signalling:

    • Ligand impacts transcription or enzymatic activity directly without receptors.

8.2 Typical Ligands

  • Examples of Ligands by Type:

    • Steroid Hormones: Estrogen, cortisol (before secretion).

    • Growth Factors: Intracellular Angiotensin II.

    • Second Messengers: Ca²⁺, IP₃.

    • Metabolites: ROS, NAD⁺.

8.3 Biological Roles

  • Key Roles:

    • Fine-tuning of gene expression

    • Rapid stress responses

    • Cell fate commitment

  • Examples of Pathways:

    • Steroid hormone signalling before secretion

    • Intracellular angiotensin II influencing cardiac hypertrophy

    • Intracellular Ca²⁺ signalling.

9. Autocrine Signalling

  • Definition: Autocrine signalling involves a cell secreting a ligand, expressing the receptor for that ligand, and then responding to its own signal, creating self-stimulation loops.

9.1 Mechanisms

  1. Ligand Synthesis and Secretion

  2. Receptor Binding on Cell-Surface

  3. Activation of Canonical Signalling Pathways

  4. Positive Feedback Mechanisms

9.2 Typical Ligands

  • Examples of Ligands by Type:

    • Growth Factors: EGF, PDGF.

    • Cytokines: IL-2, IL-6.

    • Chemokines: CXCL8.

    • Lipid Mediators: Prostaglandins.

9.3 Biological Roles

  • Functions:

    • Cell survival

    • Proliferation

    • Reinforcement of differentiation

  • Examples:

    • IL-2 autocrine loop in activated T cells

    • EGF signalling in epithelial repair

    • Autocrine growth factor loops in cancer cells.

10. Paracrine Signalling

  • Definition: Paracrine signalling involves a cell secreting ligands that act on nearby cells within the same tissue or microenvironment.

10.1 Mechanisms

  1. Ligand Secretion: The cell produces and secretes ligands.

  2. Diffusion: Ligands diffuse through interstitial space.

  3. Binding: Ligands bind to receptors on neighbouring cells.

  4. Activation: Activation of intracellular pathways.

10.2 Typical Ligands

  • Examples of Ligands by Type:

    • Growth Factors: FGF, VEGF.

    • Morphogens: Wnt, Hedgehog.

    • Cytokines: TNF-α.

    • Gaseous Signals: Nitric oxide (NO) diffuses freely.

10.3 Biological Roles

  • Functions:

    • Tissue patterning

    • Angiogenesis

    • Immune responses

    • Wound healing

  • Examples:

    • VEGF from hypoxic cells acting on endothelial cells.

    • NO from endothelium affecting smooth muscle.

    • Wnt gradients in developmental contexts.

11. Comparing Intracrine, Autocrine, and Paracrine Signalling

Feature

Intracrine

Autocrine

Paracrine

Signal Leaves Cell

No

Yes

Yes

Target Cell

Same cell

Same cell

Nearby cells

Requires Receptor

Sometimes

Yes

Yes

Range

Intracellular

Very short

Short

Signal Dilution

None

Low

Moderate

Typical Ligands

Steroids, Ca²⁺

Growth factors

Morphogens

11.1 Integration in Real Tissues

  • Multi-Mode Usage: Real tissues often utilize several signalling modes simultaneously.

    • Example: In intestinal crypts:

    • Wnt → paracrine function related to stemness.

    • Notch → juxtacrine role in fate determination.

    • EGFR → autocrine role supporting survival.

    • Ca²⁺ → intracrine signalling ensuring rapid responses.

12. Big-Picture Overview of Wnt Signalling

  • Functionality: Wnt signalling determines whether β-catenin is degraded or allowed access to the nucleus, which triggers significant transcriptional outcomes.

  • Wnt Signalling Responsibilities:

    • Cell fate determination

    • Proliferation

    • Polarity

    • Migration

    • Stem cell maintenance

  • Importance: Critical for embryonic development and tissue homeostasis. Wnt signalling primarily operates via paracrine signalling but can also act autocrine in cancer and interacts strongly with contact-dependent systems (such as adherens junctions and Notch).

12.1 Core Components of the Wnt System

  • Wnt Ligands: Approximately 19 Wnt proteins in humans.

    • Characteristics:

    • Lipid-modified through post-translational processes (palmitoylation by Porcupine).

    • Hydrophobicity results in short-range signalling.

    • Frequently bind in the ECM to facilitate gradient formation.

  • Examples of Wnt Ligands:

    • Wnt1, Wnt3a (canonical pathways)

    • Wnt5a, Wnt11 (non-canonical pathways).

12.2 Canonical Wnt / β-Catenin Pathway

  • Understanding the OFF State (absence of Wnt ligand):

    • Destruction Complex:

    • β-catenin consistently degraded in the absence of Wnt ligands via a destruction complex composed of:

      • Axin (scaffold)

      • APC (Adenomatous Polyposis Coli)

      • GSK3β

      • CK1

    • Mechanism of Degradation:

    1. β-catenin binds to the destruction complex.

    2. CK1 phosphorylates β-catenin to prime it for subsequent phosphorylation.

    3. GSK3β adds additional phosphates.

    4. Phosphorylated β-catenin is recognized by β-TrCP, leading to ubiquitination and degradation.

    • Result: Cytoplasmic β-catenin levels remain low, inhibiting Wnt target gene activation as TCF/LEF binds DNA with repressors (e.g., Groucho).

12.3 ON State (presence of Wnt ligand)

  • Process for Wnt Activation:

    1. Ligand Binding: Wnt binds to Frizzled and LRP5/6 receptors, forming a heterodimer complex.

    2. Dishevelled Activation: Dishevelled (DVL) becomes recruited to Frizzled and activates signalling cascades.

    3. Dismantling Destruction Complex:

    • Axin binds phosphorylated LRP6, leading to the suppression of GSK3β activity, preventing β-catenin degradation.

    1. β-Catenin Stabilization:

    • β-catenin accumulates in the cytoplasm, enters the nucleus.

    1. Transcriptional Activation:

    • β-catenin binds TCF/LEF, displacing repressors, and recruits co-activators, ultimately activating Wnt target genes.

12.4 Non-Canonical Wnt Pathways (β-catenin Independent)

  • Function: Control various cellular movements and tissue organization without contributing to proliferation.

  • Types of Non-Canonical Pathways:

    • Wnt/Planar Cell Polarity (PCP) Pathway:

    1. Wnt binds to Frizzled.

    2. Activates Rho/Rac to lead to cytoskeletal rearrangement for cellular polarization.

    • Functions:

      • Neural tube closure

      • Convergent extension

      • Directed cell migration

    • Wnt/Ca²⁺ Pathway:

    1. Wnt binds Frizzled, activating G proteins.

    2. Triggering Ca²⁺ release from the endoplasmic reticulum (ER) activating downstream pathways.

    • Functions:

      • Axon guidance

      • Heart development

      • Cell migration.

13. Big-Picture Overview of TGF-β Signalling

  • Signalling Type: Paracrine and autocrine processes governed by the Transforming Growth Factor-β (TGF-β) superfamily which regulates:

    • Cell proliferation

    • Differentiation

    • Apoptosis

    • ECM production

    • Immune regulation

    • Epithelial-mesenchymal transition (EMT).

13.1 Ligand Synthesis and Activation

  • Latent TGF-β Complex:

    • Composed of:

    • TGF-β dimer

    • LAP (latency-associated peptide)

    • LTBP (latent TGF-β-binding protein)

    • Activation Mechanisms:

    • Through proteolysis, integrin-mediated mechanical pulling, and changes in pH or ROS levels, dictating the spatial and temporal context of signalling.

13.2 TGF-β Receptors

  • Receptor Type Overview:

    • Type II Receptor (TβRII): Constitutively active kinase that binds ligand first.

    • Type I Receptor (TβRI): Also known as ALK5; recruited by TβRII; phosphorylated to become active.

  • Receptor Complex: Forms a heterotetrameric complex (2 Type II and 2 Type I receptors).

13.3 Canonical TGF-β/SMAD Pathway

  • Signalling Cascade Overview:

  1. Ligand binding with active TGF-β to TβRII.

  2. TβRII phosphorylates TβRI, activating its serine/threonine kinase function.

  3. TβRI phosphorylates SMAD2 and SMAD3 (R-SMADs).

  4. Formation of SMAD complex with phosphorylated R-SMADs binding SMAD4 (co-SMAD).

  5. Nuclear translocation of SMAD complex to initiate transcription.

13.4 Negative Regulation of TGF-β Signalling

  • Mechanisms of Regulation:

    • Inhibitory SMADs (e.g., SMAD6, SMAD7): Block receptor-SMAD interactions and recruit ubiquitin ligases for receptor degradation.

    • Extracellular Antagonists: Decorin and follistatin hinder TGF-β signalling pathways by inhibiting effects on target cells.

14. Big-Picture Overview of Hedgehog Signalling

  • Signalling Type: Primarily paracrine, Hedgehog signalling regulates the activity of GLI transcription factors, which can serve as activators or repressors based on Hedgehog ligand presence.

  • Functions of Hedgehog Signalling:

    • Embryonic patterning

    • Cell fate specification

    • Proliferation

    • Stem cell maintenance

  • Key Functions in Development: Important for anterior-posterior patterning, neural tube and limb development; often quiescent post-development but may reactivate during tissue repair or cancer.

14.1 Signal Transduction Hub: Primary Cilium

  • Hedgehog signalling occurs in the primary cilium, serving as a central signalling organelle that facilitates pathway component trafficking in and out.

14.2 Canonical Hedgehog Pathway (OFF State)

  • States Without Hedgehog Ligand:

    1. PTCH Inhibition: PTCH protein inhibits Smoothened (SMO) within the cilium, preventing its accumulation.

    2. GLI Processing: GLI protein binds SUFU and undergoes sequential phosphorylation, leading to GLI-R formation (repressor).

14.3 Activation of Hedgehog Pathway (ON State)

  • Ligand Presence (Hedgehog Ligand Present):

    1. Ligand binding to PTCH.

    2. PTCH localization dynamics leading to SMO activation in the cilium.

    3. GLI Activation: Inhibition of SUFU and GLI-A (active form of GLI) translocating into the nucleus for gene transcription.

  • Key Target Genes: Examples include GLI1 (positive feedback), PTCH1 (negative feedback), CCND1 (cell cycle and proliferation), MYC (growth), BCL2 (survival).

14.4 Gradient Interpretation and Morphogen Function

  • Through concentration gradients, Hedgehog exhibits morphogen properties that are dependent on duration and strength. Example observed in the neural tube concerning concentration-dependent effects.

15. Big-Picture Overview of JAK–STAT Signalling

  • Function: Acts primarily in paracrine or autocrine fashion, with possible endocrine activity; ligand binding activates JAK tyrosine kinases which phosphorylates STATs.

  • Defining Features:

    • Very rapid response

    • Linear signal transduction with few intermediates

    • Direct membrane receptor-to-nucleus linking

15.2 JAK-STAT Receptor Mechanics

  • Common Characteristics: Single-pass transmembrane proteins lacking intrinsic enzymatic activity, existing as pre-formed dimers or ligand-induced receptor dimers.

15.5 Canonical JAK-STAT Pathway Steps

  1. Ligand Binding: Cytokine binding induces receptor dimerization or orientation changes.

  2. JAK Activation: JAKs come into proximity, facilitating trans-phosphorylation.

  3. Receptor Activation: Creation of docking sites through the phosphorylation of receptor tails by JAK.

  4. STAT Recruitment: STATs can bind to phosphotyrosines through their SH2 domains.

  5. STAT Phosphorylation: Additional phosphorylation by JAK on STAT proteins, enabling their dimerization.

  6. Nuclear Translocation: STAT dimers translocate into the nucleus where they bind to DNA.

Signal Termination and Regulation

  • Regulators:

    • SOCS Proteins: Inhibit JAK activity and promote degradation.

    • PIAS Proteins: Block STAT DNA binding.

    • Phosphatases: Dephosphorylate JAKs and STATs, leading to signal attenuation.

16. Integration of Signalling

  • Unified Signalling Story: From patterning to fate determination, maintenance, and response.

  • Framework Concept: Tissues answer sequential questions regarding positional identity, proliferation, differentiation fate, remodeling, and response to environmental stimuli.

16.1 Hedgehog Signalling

  • Function: Places cellular positioning in the body based on morphogen gradient and transcription outputs.

  • Example: Dictates neural tube patterning and limb anterior-posterior axis establishment.

16.2 Wnt Signalling

  • Function: Maintains stemness and proliferative capacity while cells adapt positional information.

  • Examples: Wnt pathways enhancing stability of β-catenin emphasize cell cycle entry and maintain stem cell niches within defined areas.

16.3 Notch Signalling

  • Function: Refines cell fate decisions based on neighbour identity.

  • Examples: Physically close cells with Notch and Delta expressing differential cell types that govern developmental lineage.

16.4 TGF-β Signalling

  • Function: Regulates growth termination, specialization, and maintenance of tissue integrity as stated by cellular context and needs.

  • Examples: TGF-β can antagonize Wnt-induced proliferation and complement Notch during definitive differentiation pathways.

16.5 JAK-STAT Signalling

  • Function: Extraordinarily rapid response to immediate stimuli, governing cellular adaptations based on environmental promptings.

17. Long-Distance (Endocrine) Signalling

  • Definition: Occurs when cells (endocrine glands or specialized tissues) release hormones into the bloodstream which act at distant target cells with appropriate receptors to execute physiological effects.

Key Features:

  • Long range of action across the body, yet specific through receptor interactions, almost always occurring at low ligand concentrations with effects that are slower but sustained.

17.1 Hormone Transport and Specificity

  • Transport Mechanisms:

    • Free Hormone: Active and available to exert effects, while Bound Hormone: Serves as a storage reservoir.

    • Carrier Proteins: Include Albumin, SHBG, TBG, etc.

  • Target Specificity Factors:

    • The responsiveness relates primarily to receptor expression, isoforms, and balancing local signalling integration which allow for diverse tissue responses based on similar hormones.

17.2 Time-Scale Hierarchy of Signalling

Time Scale

Signalling Mode

Seconds

Neurotransmitters

Minutes

GPCRs, JAK-STAT

Hours

Wnt, Notch

Days

TGF-β, steroid hormones

Weeks

Morphogen-driven tissue changes