Cell biology lecture 15

What is a signalsome? 🧩

A signalsome is the unique set of receptors, G proteins, enzymes, and targets expressed in a cell that determines how it responds to signals.

Different cells = different outcomes.

Why can the same hormone cause different effects in different tissues? 🔄

Because each tissue expresses different receptor subtypes and signalling proteins, so the same signal is processed differently.

Which scientist first distinguished α and β adrenoceptors? 🧠

Ahlquist (1948), which laid the foundation for receptor subtypes and selective drugs like β-blockers.

Which G protein do β-adrenoceptors couple to? ⚡

Gs protein, which stimulates adenylyl cyclase and increases cAMP.

Which G protein do α₂-adrenoceptors couple to? ⛔

Gi/Go, which inhibits adenylyl cyclase and reduces cAMP while also releasing βγ subunits.

Which G protein do α₁-adrenoceptors couple to? 💥

Gq, which activates PLCβ leading to IP₃ and DAG production.

What is the “push–pull” regulation of cAMP? ⚖️

cAMP levels are controlled by a balance between Gs stimulation and Gi inhibition of adenylyl cyclase, allowing fine-tuned control.

Describe the β-adrenergic (Gs) signalling pathway 📈

Adrenaline binds β-receptor → Gs activated → adenylyl cyclase activated → ATP converted to cAMP → cAMP activates PKA and EPAC → target phosphorylation.

What is cAMP? 🔑

cAMP is a second messenger made from ATP that activates PKA, EPAC, and cyclic nucleotide-gated channels.

What enzyme makes cAMP? 🏭

Adenylyl cyclase converts ATP into cAMP.

What enzyme breaks down cAMP? 🧹

Phosphodiesterase (PDE) degrades cAMP into AMP to terminate signalling.

How does cholera toxin affect signalling? ☠️

Cholera toxin locks Gs in its active state, causing constant cAMP production and massive water secretion → severe diarrhoea.

How does pertussis toxin affect signalling? 🚫

Pertussis toxin prevents Gi from being activated, blocking its inhibitory effects and βγ signalling.

Why are cholera and pertussis toxins useful experimentally? 🧪

They help distinguish whether a pathway depends on Gs or Gi signalling.

Why are βγ subunits important? 🔓

βγ subunits actively signal by opening ion channels (like GIRK) and activating pathways such as PI3K.

What are GIRK channels and how are they activated? 🔋

GIRK channels are K⁺ channels opened by βγ subunits from Gi/Go-coupled receptors, causing hyperpolarisation.

What is hyperpolarisation? 📉

Hyperpolarisation makes the cell more negative, reducing the chance of firing an action potential.

What is autaptic signalling? 🔁

Autaptic signalling occurs when a neuron releases neurotransmitter that feeds back onto its own receptors to limit further release of that neurotransmitter.

How do Gi/Go-coupled receptors reduce neurotransmitter release? 🛑

They reduce Ca²⁺ entry and activate GIRK channels, lowering neuronal excitability.

Describe the α₁-adrenergic (Gq) signalling pathway 🧬

Ligand binds α₁-receptor → Gq activated → PLCβ activated → PIP₂ split into IP₃ + DAG → IP₃ releases Ca²⁺ → DAG activates PKC.

What is PIP₂? 🧱

PIP₂ is a membrane phospholipid that is cleaved by PLC to produce IP₃ and DAG.

What does IP₃ do? 💧

IP₃ binds to receptors on the ER and triggers Ca²⁺ release into the cytosol.

What does DAG do? 🔥

DAG stays in the membrane and activates protein kinase C (PKC).

What is the IP₃ receptor? 🚪

A ligand-gated Ca²⁺ channel on the ER that releases Ca²⁺ when IP₃ binds.

What are the three PKC classes? 🧠

cPKC (Ca²⁺ + DAG), nPKC (DAG only), aPKC (neither Ca²⁺ nor DAG).

How is PKC kept inactive? 🔒

A pseudosubstrate blocks the active site until DAG (and Ca²⁺ for cPKC) bind.

What do RACK proteins do? 🧲

RACKs anchor activated PKC to specific locations in the cell for precise signalling.

How does α₁ signalling cause smooth muscle contraction? 💪

Ca²⁺ binds calmodulin → activates MLCK → myosin light chain phosphorylated → contraction.

How does adrenaline regulate glycogen breakdown in the liver? 🍬

β-receptors raise cAMP and α₁-receptors raise Ca²⁺, both activating phosphorylase kinase to promote glycogenolysis.

What are the main effectors of cAMP? 🎯

PKA, EPAC, and cyclic nucleotide-gated channels.

What is EPAC? 🔄

EPAC is a cAMP-activated GEF that activates RAP, linking cAMP to secretion, adhesion, and junction control.

Why does EPAC respond at higher cAMP levels than PKA? 📊

EPAC has lower affinity for cAMP, allowing graded signalling responses.

How is GPCR signalling switched off? 🛑

Gα hydrolyses GTP, receptors are phosphorylated and bind arrestin, cAMP is degraded by PDEs, and phosphatases remove phosphates.

What is the final take-home message of this lecture? 🎓

Signalling specificity comes from the cell’s signalsome, not from the hormone itself.