Lecture 16 (Grimm) Flashcards
Introduction into Signal transduction pathways
What is meant by Signal Transduction?
-
Definition of Signal Transduction:
- The process by which signals from the extracellular environment and intracellular developmental processes are amplified and converted into specific cellular responses -
Purpose of Signal Transduction:
- The molecular events of a cell for a coordinated and specific response to the environment and development. -
Conversion of Signals:
- Conversion of signal to chemical forms that are specific and capable of activating cellular responses. -
Receptor Activation:
- Binding of a signal to the receptor leads to structural changes of the perceiving molecule (receptor) resulting in submission of the signal. -
Amplification through Cascades:
- Amplification occurs through interactions of multiple components to ensure a rapid and specific response, even when the initial signal level is low.
Mechanisms of intercellular Signalling
Mechanisms of intercellular Signalling
- Signaling between cells through chemical signaling
In bacterial cell-to-cell exchange:
- Via intercellular messengers: Ligands are released, diffuse freely between cells, and are recognized by receptors.
- Via surface proteins: Protein-protein interactions on the cell surface enable direct contact.
In animals and plants:
- via Gap junctions: Channels that directly connect the cytoplasms of adjacent animal cells, allowing small molecules and ions to pass freely between them.
- via Plasmodesmata: Microscopic channels in plant cell walls that link neighboring cells’ cytoplasms, enabling the exchange of substances like nutrients, signaling molecules, and ions.
Intercellular Signalling
Short-distance signaling
Short-Distance Signaling
- Contact-dependent signaling: Requires cells to be in direct membrane-membrane contact.
- Paracrine signaling: depends on local mediators that are
released into the extracellular space and act on neighboring cells.
- Autocrine Signaling: Cells producing signals that they themselves respond to. Cancer cells, for example, often produce extracellular signals that stimulate their own survival and proliferation.
- Synaptic signaling: Is performed by neurons that transmit signals electrically along their axons and release neurotransmitters (chemical signal) at chemical synapses, which are often located far away from the neuronal cell body.
Intercellular Signalling
Long distance signalling
Long distance signalling
- Endocrine signaling: Endocrine cells secrete signal molecules, called hormones, into the bloodstream. The blood carries the molecules far and wide, allowing them to act on target cells that may lie almost anywhere in the body
- Phytohormones in plants are transmitted polar (direct from top to bottom) through the vascular tissue (xylem or sieve elements) or through parenchyma cells
Light-Dependent Flowering in Plants
Light-Dependent Flowering in Plants
- Conversion from the formation of leaves to flowering organs.
- Signal for flowering development is the recognition of light.
- Plants flower under either long-day (LD) or short-day (SD) conditions.
- Phytochrome (PHYB) and other photoreceptors (e.g., PHYA, CRY) are activated to induce the signaling pathway.
- The signal is recognized in the leaves, where clock genes regulate the production of specific proteins (e.g., FT protein for LD plants or Hd3a protein for SD plants).
- The protein is transferred through the phloem to the shoot apex (floral meristem) to trigger flowering.
Events in a Signal Transduction
Events in a signal transduction
- For the receiving cell there are 3 stages in the signaling process: Reception, Transduction and Cell Response
The Signal – The first messenger
Events in a Signal Transduction
The Signal – The first messenger
- A signal typically interacts with multiple target cells
- It binds to different receptors, triggering distinct transduction pathways.
- These pathways lead to varying responses depending on the target cell or tissue.
Signals can function in various ways:
- Synergistically or cooperatively, enhancing the response.
- Antagonistically or inhibitory, suppressing the response.
- Hierarchically, prioritizing certain pathways over others.
Reception of a signal
Events in a Signal Transduction (Types of Receptors)
Reception of a signal
- The targeted cell has a receptor molecule complementary to the signal molecule or ligand that triggers a change in the receptor molecule
- Cytosolic or Nuclear Receptors: Amphiphilic/Lipophilic ligand/ signals (e.g. steroid hormones, phytochrome, gas) cross the membrane freely and target receptors in the cytoplasm, this often activates genes
- Cell membrane bound Receptors: Hydrophilic/lipophobic ligand can not enter cell, Fast response
Soluble receptors inside cells
Example
Soluble receptors inside cells (Phytochrome)
- Phytochrome is a light-sensitive receptor that exists in two forms: inactive (Pr) and active (Pfr)
- In the presence of red light, Pr is converted to the active Pfr form, which can move into the nucleus.
- Inside the nucleus, the active phytochrome interacts with transcription factors (e.g., PIFs - Phytochrome Interacting Factors).
- This interaction regulates the transcription of light-inducible genes, promoting processes like photomorphogenesis or inhibiting shade-induced genes.
- In the absence of light, Pfr is converted back to the inactive Pr form, or it becomes degraded, effectively inactivating the pathway.
- The ratio of red to far-red light (R/FR ratio) is crucial in determining whether phytochrome is active or inactive, influencing plant responses to sunlight or shade.
Transduction of a signal
Events in a Signal Transduction
Transduction of a signal
- Signal transduction converts changes in a receptor into a form that can trigger a cellular response.
- This process often involves a series of steps, forming a signal transduction pathway that alters and amplifies the initial signal.
- Intracellular signaling molecules, including second messengers, relay signals from cell-surface receptors to the cell’s interior.
- Second messengers (e.g., cyclic AMP, Ca²⁺, and diacylglycerol) are small molecules generated in large amounts following receptor activation.
- Water-soluble second messengers (e.g., cAMP, Ca²⁺) diffuse through the cytosol.
- Lipid-soluble second messengers (e.g., DAG) diffuse within the plasma membrane.
- These molecules spread the signal by binding to and altering the activity of specific signaling or effector proteins, ultimately modifying the cell’s behavior.
Response
Events in a Signal Transduction
Cellular Response
- The Response can be many different cellular activities, such as activation of a certain enzyme, rearrangement of the cytoskeleton or activation of specific genes
Membrane Receptor Structure & Classes
Membrane Receptor Structure
- Plasma Membrane Receptors bind signaling molecules via the extracellular domain
- Signal is transmitted through trans-membrane domain to intracellular domain
- Intracellular domain further transmits response to interior of cell, often with amplification
- Classes: Enzyme-linked Receptor, Ion-channel linked Receptor, G-protein-linked Receptor (defined by their transduction mechanism)
Membrane Receptor Classes
Enzyme-linked receptor
Enzyme-linked Receptor
- Either function as enzymes or activate associated enzymes.
- Typically single-pass transmembrane proteins with an extracellular ligand-binding site and an intracellular catalytic or enzyme-binding site.
- They exhibit structural heterogeneity compared to other receptor classes.
- Most enzyme-coupled receptors are protein kinases or associate with protein kinases.
- Activated receptors phosphorylate specific sets of proteins in the target cell.
Example of Enzyme-linked Receptor
(Histidine kinase)
Membrane Receptor Classes
Two-component-receptor: Histidine kinase
- Consisting of receptor kinase and response regulator
- Receptor kinase consists of input (receptor) and transmitter domain
- Response regulator consists of receiver- und output domain
In the signaling process:
- The receptor kinase is activated by an external signal, transferring a phosphate group from ATP to its transmitter domain.
- The phosphate group is then passed to the receiver domain of the response regulator.
- The output domain of the response regulator initiates the cellular response.
- In a multi-step system, a Histidine-phospho-transfer protein (HPt domain) serves as an intermediate, transferring the phosphate group from the receptor kinase to the response regulator.
Example of Enzyme-linked Receptor
(Tyrosine Kinase)
Membrane-bound Receptors
Tyrosine Kinase
- Protein kinases attach phosphate to the hydroxyl group of specific amino acids on the target protein
- Tyrosine Kinase phosphorylates proteins on tyrosines
- found mostly in multicellular animals
Signal transduction of the Receptor Tyrosine Kinase
1. Ligand binding and Dimerisation (Homo- or Heterodimer)
3. Autophosphorylation
4. Phosphorylated tyrosines are recognized by proteins with
recogniton domain
5. Signal transduction by phosphorylation cascade through relay proteins
6. Response, e.g. gene expression
Example of Enzyme-linked Receptor
(Receptor-like-kinases in plants)
Membrane-bound Receptors
Receptor-like-kinases in plants (RLK)
- Not structurally related but contain same 3 functional domains: a large extracellular domain, a single membrane spanning domain and a cytoplasmic domain (serine/theronine kinase function)
- After ligand binding, the receptor phosphorylates and
activates other downstream proteins
- different signalling cascades down to transcription factors
- Plant RLK are classified into subfamilies based on the structural feature of the extracellular domain
Model for CLV1 signaling
Example of Receptor-like-kinases in plants
Model for CLV1 Signaling
- CLV-WUS pathway is essential for regulating stem cell populations in plants, ensuring balanced growth and differentiation.
- CLV3 (Clavata 3): A peptide signal processed by a serine protease to create the active ligand.
- CLV1 (Clavata 1): A receptor-like kinase that binds the mature CLV3 peptide signal, along with potentially other receptors like BAM and CLV2.
- WUS (Wuschel): Encodes a transcription factor critical for maintaining stem cell identity.
Signaling Process
1. CLV3, the active ligand, binds to CLV1 and potentially interacts with receptor complexes like CLV2 and BAM to amplify or coordinate the signal.
2. Receptor activation represses POL/PLL1, positive regulators of WUS transcription, limiting WUS activity.
3. Repression of WUS reduces stem cell activity and promotes differentiation in the shoot apical meristem, maintaining proper development.
4. Simultaneously, the signal from CLV3 represses phosphatases, leading to dephosphorylation of transcription factors and activation of WUS gene expression.
Feedback Mechanism
- High WUS activity stimulates CLV3 production, creating a feedback loop that maintains the balance between stem cell proliferation and differentiation.
Ion channel-linked Receptor
Membrane Receptor Classes
Ion channel-linked Receptor
- Allow the selective passage of ions (e.g., Na⁺, K⁺, Ca²⁺, Cl⁻) across the plasma membrane.
- Highly selective, allowing specific ions to pass based on size and charge.
They can be gated (opened or closed) by various stimuli, such as:
- Ligands (ligand-gated ion channels).
- Voltage changes (voltage-gated ion channels).
- Mechanical forces (mechanically gated ion channels).
Ion flow through these channels creates electrical signals critical for processes like:
- Nerve impulse transmission.
- Muscle contraction.
- Regulation of cellular homeostasis.
G protein-coupled receptor
Structure & Function
Membrane Receptor Classes
G protein-coupled Receptor
- GPCRs are seven-α-helix-membrane-spanning receptors, with an extracellular domain for ligand binding and a cytoplasmic domain for interacting with G-proteins.
- heterotrimeric oder monomeric
- G-proteins function as signal transducers, converting external signals into intracellular responses by acting as molecular switches.
- Upon ligand binding, they are activated by GTP binding and return to an inactive state through GTP hydrolysis.
- G-proteins enhance signaling by mediating, amplifying, and ensuring precise recognition of signals by their receptors, improving accuracy and specificity in signal transduction.
- Ligands include hormones, neurotransmitters, and sensory stimuli (light, odor).
G protein-coupled receptor
The monomeric G protein
Membrane Receptor Classes
The small monomeric G protein
- A ligand binds to the receptor, causing a conformational change or dimerization that activates the receptor.
- The activated receptor phosphorylates or recruits downstream signaling molecules, including GEFs (Guanine nucleotide exchange factors).
- GEFs activate the G-protein by facilitating the release of GDP. Due to the higher cytosolic concentration of GTP, the G-protein rapidly binds GTP, becoming active.
- The GTP-bound G-protein undergoes a conformational change, particularly in its switch regions, exposing binding sites or interaction surfaces.
- This allows the activated G-protein to bind and activate downstream effectors, propagating the signal.
- GAPs (GTPase-activating proteins) inactivate the G-protein by stimulating the hydrolysis of GTP to GDP, returning the G-protein to its inactive state.
- GDI (Guanine Nucleotide Dissociation Inhibitor): Prevents GDP dissociation, keeping the G-protein in its inactive GDP-bound state and blocking activation
The heterotrimeric G protein
Heterotrimeric G Proteins
- Composed of three subunits: α (binds GTP/GDP), β, and γ.
- Act as a molecular switch, coupling integral membrane receptors (e.g., GPCRs) to target membrane-bound enzymes or ion channels.
Mechanism:
1. Signal Reception: A ligand binds to the receptor, causing a conformational change.
2. The receptor activates the G-protein by exchanging GDP for GTP on the α-subunit.
3. G-protein dissociates into an active GTP-bound α-subunit and βγ-dimer, both capable of activating downstream effectors.
4. The active G-protein subunits (α or βγ) interact with specific target proteins to propagate the signal and generate second messengers like cAMP or IP₃/DAG.
5. The intrinsic GTPase activity of the α-subunit hydrolyzes GTP to GDP, returning the G-protein to its inactive form. Regulator of G-protein signaling (RGS) proteins can accelerate this inactivation.
Phospholipase C (PLC)
Phospholipase C (PLC)
- an enzyme that plays a key role in intracellular signal transduction by cleaving membrane phospholipids.
Function:
- Hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP₂), a phospholipid in the plasma membrane, into two important second messengers:
- Inositol 1,4,5-trisphosphate (IP₃): Water-soluble, diffuses into the cytoplasm, and triggers the release of calcium ions from the endoplasmic reticulum.
- Diacylglycerol (DAG): Lipid-soluble, remains in the membrane, and activates protein kinase C (PKC).
Activation:
- PLC is activated by G-protein-coupled receptors (via Gq-type G-proteins) or receptor tyrosine kinases (via adaptor proteins).
Role in Signaling:
- Amplifies and propagates signals from extracellular stimuli like hormones, neurotransmitters, and growth factors.
- Regulates processes like cell proliferation, differentiation, and metabolism.
Eukaryotic Secondary Messengers
Eukaryotic Second messengers
- Small non-protein intracellular signaling molecules that act as intermediaries in signal transmission, generated in large amounts in response to receptor activation.
- They diffuse from their source to spread the signal within the cell and bind to specific signaling or effector proteins, altering their behavior.
Examples
- Cyclic AMP (cAMP): A water-soluble messenger that acts in pathways initiated by both G-protein-coupled receptors and receptor tyrosine kinases.
- Calcium ions (Ca²⁺): A versatile secondary messenger involved in various cellular responses.
- Diacylglycerol (DAG): A lipid-soluble messenger that diffuses within the plasma membrane.
Eukaryotic secondary messengers: Calcium
Eukaryotic secondary messengers: Calcium
Interactions of intracellular and extracellular calcium in cell-signalling
- Ca is stored in organelles and cell wall
- low content in cytoplasm
- Signal opens channels in various organelles and plasma membrane
- Ca-ATPases and Ca/H-antiporters lowers again the cytoplasmic
concentration
- The intensity of the stimulus controls the amplitude (localized or global signal) and spatial distribution of the calcium signal.
- Stronger stimuli activate additional calcium stores and sensors, leading to more widespread and amplified cellular responses.
How can the specific physiological responses be created by calcium ions?
- Duration of the signal action, duration of the elevated levels of calcium
- Number of oscillations
- Number of receptors, channels and Ca²+-ATPases
- Spatial specificity : at a certain site, in a certain subcompartment of the cell, spatial distribution of receptors, channels and Ca²+-ATPases
