week 6 Cell Compartmentalisation Flashcards
Define compartmentalisation in the context of cell signalling and explain why it is important.
✅ Model Answer:
Definition:
Compartmentalisation is the restriction of signalling events to specific regions within the cell, ensuring selective and appropriate responses to stimuli.
Importance:
Prevents widespread activation of non-targeted pathways.
Allows specificity: one stimulus triggers a specific subset of cellular responses.
Enables localized control despite global changes in second messengers like cAMP and calcium.
✏️ Question 2:
Describe how mitochondria contribute to signal compartmentalisation.
✅ Model Answer:
* Mitochondria regulate cytosolic calcium by:
o Uptake of calcium into the mitochondrial matrix via calcium uniporters.
o Release of calcium back into the cytosol, influencing local calcium levels.
* Mitochondria are spatially positioned in regions critical for processes like exocytosis.
* They maintain localized calcium environments that control secretion events (e.g., in pancreatic beta cells).
Explain the role of the nucleus in compartmentalisation of cellular signals.
✅ Model Answer:
* Nucleus compartmentalises transcriptional regulation:
o Signals like phosphorylation of transcription factors allow nuclear import.
o Only once inside the nucleus can factors influence gene expression.
* Movement through the nuclear pore is essential — localization controls whether or not gene transcription occurs.
✏️ Question 4:
How is the Golgi apparatus involved in cell signalling compartmentalisation?
✅ Model Answer:
* Traditionally associated with protein sorting and trafficking.
* Now recognized as:
o A calcium signalling center.
o Organizing specific signal cascades by controlling localized calcium stores and vesicle movement.
Discuss how the endoplasmic reticulum (ER) contributes to calcium signalling.
✅ Model Answer:
* The ER acts as a major intracellular calcium store.
* Release mechanisms:
o IP₃ receptors and ryanodine receptors allow calcium efflux into the cytosol upon stimulation.
* Localized release results in spatially restricted calcium spikes affecting processes like secretion and gene activation.
✏️ Question 6:
How do protein-protein interactions contribute to signalling compartmentalisation? Provide examples.
✅ Model Answer:
* Protein clustering brings enzymes and targets into proximity:
o Insulin receptor pathway:
Dimerization of receptors leads to downstream signalling cascades (e.g., PI3K activation).
o Integrin signalling:
Clustering at the leading edge during cell migration activates focal adhesion complexes.
* Co-localisation is critical:
Without physical proximity, signalling events cannot proceed.
What is the role of integrins in compartmentalised signalling?
✅ Model Answer:
* Integrins are transmembrane receptors linking the extracellular matrix to intracellular cytoskeleton and signalling machinery.
* Their activation clusters associated proteins (e.g., talin, kindlin, vinculin) into focal adhesion complexes.
* This clustering triggers localized signalling regulating migration, adhesion, and cytoskeletal organization.
Describe the classical global model of cAMP signalling and how recent findings challenge it.
✅ Model Answer:
* Classical View:
cAMP, once produced (e.g., via GPCR activation and adenylyl cyclase), freely diffuses through the cytosol, activating targets like PKA throughout the cell.
* New Understanding:
o cAMP signalling is spatially restricted.
o Phosphodiesterases (PDEs) degrade cAMP locally.
o AKAPs (A-kinase anchoring proteins) tether PKA to specific regions ensuring local responses.
What is the function of phosphodiesterases (PDEs) in cAMP signalling compartmentalisation?
✅ Model Answer:
* PDEs hydrolyze cAMP into inactive AMP.
* Localized PDE activity:
o Creates microdomains with low cAMP concentration.
o Prevents undesired activation of PKA or EPAC (exchange protein activated by cAMP) elsewhere.
* Fine-tunes responses to specific regions despite a global cAMP increase.
Explain how AKAPs contribute to the spatial regulation of cAMP responses.
✅ Model Answer:
* AKAPs anchor PKA near its substrates and sources of cAMP.
* Ensure:
o Rapid and targeted activation of PKA at specific sites.
o Increased signalling specificity.
* Different AKAPs are localized in distinct cellular compartments (e.g., near ion channels, in mitochondria).
Compare global and local calcium signalling within cells.
✅ Model Answer:
* Global Calcium Signalling:
o Calcium levels rise throughout the entire cytosol.
o Leads to wide-ranging effects like secretion, contraction, gene transcription.
* Local Calcium Signalling:
o Restricted to small subcellular regions.
o Enables localized control over specific processes (e.g., localized exocytosis, focal adhesion regulation).
How are calcium signals confined despite calcium’s ability to diffuse rapidly?
✅ Model Answer:
* Cells control calcium levels via:
o Calcium pumps (SERCA into ER, PMCA out of the cell).
o Calcium buffers (proteins binding free calcium).
o Localized release mechanisms (e.g., microdomains near IP₃ receptors or ryanodine receptors).
* Ensures spatial restriction even when global diffusion is possible.
What are calcium oscillations, and why are they important?
✅ Model Answer:
* Calcium oscillations are rhythmic rises and falls in intracellular calcium concentration.
* Important because:
o Frequency and amplitude encode information.
o Different frequencies selectively activate different downstream targets (e.g., enzymes, transcription factors like CaMKII).
How does CaMKII decode calcium oscillation frequency?
✅ Model Answer:
* CaMKII activation is:
o Dependent on cumulative calcium signals.
o If oscillations are rapid enough, residual CaMKII activation persists between calcium spikes, leading to build-up.
* Result:
High-frequency oscillations cause strong, sustained CaMKII activation, while low-frequency ones may not.
Summarise how organelles, proteins, and second messengers together achieve signalling compartmentalisation.
✅ Model Answer:
* Organelles:
Localize signalling reactions internally (e.g., ER stores calcium, mitochondria regulate calcium near secretion sites).
* Protein-Protein Interactions:
Cluster enzymes, receptors, and substrates spatially to allow efficient signalling (e.g., insulin receptor cascade, integrin signalling).
* Second Messengers:
Although diffusible, mechanisms like PDE degradation and anchoring proteins like AKAPs confine their effects to specific regions.
* Outcome:
Spatial and temporal precision in signalling ensures appropriate, highly specific cellular responses.
What is organelle signalling, and how does it contribute to cellular compartmentalisation? Provide examples.
✅ Model Answer:
* Organelle signalling refers to the generation and control of signals within specific membrane-bound compartments, allowing localized responses distinct from the cytosolic environment.
* Examples:
o Mitochondria:
Control local calcium uptake and release.
Influence cytosolic calcium concentration near secretion sites.
Key in energy metabolism (ATP production) and apoptosis signalling.
o Nucleus:
Compartment for transcription factor activity (e.g., STATs, NF-κB).
Nuclear entry of activated factors triggers gene expression changes.
o Endoplasmic Reticulum (ER):
Stores intracellular calcium.
Calcium release through IP₃ or ryanodine receptors initiates localized cytosolic calcium signalling.
o Golgi Apparatus:
Organises protein trafficking and acts as a calcium store for secretory pathways.
Explain how mitochondria control localised calcium signalling in cells.
✅ Model Answer:
* Mitochondria take up cytosolic calcium through the mitochondrial calcium uniporter.
* They locally buffer calcium elevations near exocytic sites, fine-tuning secretion responses.
* Spatial positioning of mitochondria ensures localized control of calcium signalling rather than global calcium flooding the entire cytosol.
Describe protein-protein interactions as a mechanism of signalling compartmentalisation with examples.
✅ Model Answer:
* Protein-protein interactions allow clustering of receptors, enzymes, and substrates into defined microdomains, ensuring signals occur only when components are spatially and physically close.
* Examples:
o Insulin receptor pathway:
Receptor dimerisation activates cross-phosphorylation and downstream pathways (e.g., PI3K → Akt).
All proteins must be in proximity at the plasma membrane.
o Integrin clustering:
At focal adhesions, integrins bind extracellular matrix and cluster intracellular proteins like talin, paxillin, and vinculin.
Creates a localised hub for actin cytoskeleton remodelling and cell migration.
o A-Kinase Anchoring Proteins (AKAPs):
Tether PKA near specific substrates to ensure precise activation in response to cAMP.
Why is protein co-localisation important for effective signalling?
✅ Model Answer:
* Proteins such as receptors, kinases, and effectors must be close to interact efficiently.
* Without spatial proximity:
o Signals would fail to propagate.
o Activation would be inefficient or impossible.
* Co-localisation creates signalosomes (multiprotein complexes) enabling specificity and speed in cellular responses.
How is second messenger signalling compartmentalised within cells despite their diffusible nature?
✅ Model Answer:
* Although second messengers like cAMP and calcium diffuse easily, cells restrict their effects spatially by:
o Localized production:
Adenylyl cyclases for cAMP production can be membrane-restricted.
o Localized degradation:
Phosphodiesterases (PDEs) degrade cAMP in specific regions.
o Scaffolding proteins (e.g., AKAPs):
Tether second messenger targets like PKA near production sites.
* This allows region-specific activation even when the second messenger is globally elevated.
What is the role of phosphodiesterases (PDEs) in cAMP signalling spatial regulation?
✅ Model Answer:
* PDEs hydrolyse cAMP into AMP.
* Localized PDE activity:
o Prevents uncontrolled activation of cAMP targets like PKA.
o Creates microdomains of low cAMP concentration.
* PDEs thus shape the spatial and temporal dynamics of cAMP responses.
Describe the concept of calcium microdomains and their significance in cellular signalling.
✅ Model Answer:
* Calcium microdomains are regions of locally high calcium concentration near release sites (e.g., ER membranes or plasma membrane calcium channels).
* Important for:
o Specific activation of calcium-sensitive processes (e.g., secretion, muscle contraction) without globally raising cytosolic calcium levels.
* Maintained by:
o Rapid calcium buffering.
o Calcium pumps and exchangers.
o Spatial organisation of calcium release channels (IP₃Rs, RyRs).
Explain how calcium oscillations can encode information.
✅ Model Answer:
* Calcium signals often oscillate in frequency and amplitude.
* Frequency encoding:
o Low-frequency oscillations may activate certain transcription factors.
o High-frequency oscillations more efficiently activate enzymes like CaMKII.
* Different cellular responses are triggered depending on the oscillation pattern.
