Cell Signaling: GPCR-cAMP, Ca²⁺, and Pathway Regulation

GPCR and cAMP signaling

• First messenger (e.g., epinephrine) binds to a G protein-coupled receptor (GPCR); GDP on the G protein is exchanged for GTP, activating the G protein. The active G protein then activates adenylyl cyclase.

• Adenylyl cyclase converts ATP to cyclic AMP (cAMP); the rise in cytosolic cAMP activates protein kinase A (PKA), which phosphorylates downstream targets to induce cellular responses.

• cAMP is short-lived: phosphodiesterase converts it to AMP; a new signaling event is needed to raise cAMP again.

• Amplification and economy of signaling:

  • One receptor can activate approximately $100$ molecules of G protein.
  • Each activated enzyme in the cascade can act on many substrates, leading to large downstream effects.
  • An activated adenylyl cyclase can produce roughly $100$ molecules of CAMP.
  • Each molecule of protein kinase A can phosphorylate about $10$ molecules of the next kinase.
  • The pathway can result in the release of hundreds of millions of glucose units from glycogen (≈$10^8$ molecules).

• Cholera toxin example:

  • Cholera toxin locks the G protein in its active state, causing continuous activation of adenylyl cyclase and sustained high levels of CAMP, leading to massive salt and water secretion into the intestines and severe diarrhea.

• NO/cGMP pathway and Viagra:

  • NO diffuses into smooth muscle cells and stimulates production of cyclic GMP (cGMP), a second messenger that relaxes smooth muscle.
  • A drug that inhibits cGMP hydrolysis (PDE5 inhibitor) prolongs the signal, increasing blood flow to the penis (Viagra).

• Calcium (Ca²⁺) as a second messenger:

  • Ca²⁺ is widely used as a second messenger in both animal and plant cells; cytosolic Ca²⁺ rises trigger various responses (muscle contraction, exocytosis, cell division, greening in plants).
  • Resting cytosolic Ca²⁺ is very low; extracellular fluid and organelles (ER, mitochondria) maintain high Ca²⁺ levels elsewhere. Outside the cytosol can be >$10^4$× higher than in the cytosol.
  • Ca²⁺ is pumped out of the cytosol and into the ER (and sometimes into mitochondria) by Ca²⁺-ATPases; mitochondrial pumps also help sequester Ca²⁺ during signaling.
  • Two other second messengers, IP₃ and DAG, are produced by cleavage of PIP₂ by phospholipase C (PLC). IP₃ triggers Ca²⁺ release from the ER via IP₃-gated Ca²⁺ channels; DAG remains in the membrane and, with Ca²⁺, can activate protein kinase C.

• IP₃/DAG signaling details:

  • Signaling molecule binds GPCR or receptor tyrosine kinase, activating PLC.
  • PLC cleaves PIP₂ into DAG and IP₃.
  • IP₃ diffuses to ER and opens Ca²⁺ channels, increasing cytosolic Ca²⁺; Ca²⁺ activates downstream pathways. DAG activates PKC in conjunction with Ca²⁺.

Nuclear and cytoplasmic responses (concept 11.4)

• Final output: signaling pathways regulate either nuclear gene expression or cytoplasmic activities.
• Nuclear responses:
  • Activated kinases can enter the nucleus and activate transcription factors, turning specific genes on or off and producing target mRNAs which translate into proteins.
• Cytoplasmic responses:
  • Signaling can modulate enzymes or ion channels directly, altering cellular metabolism or transport processes (e.g., glycogen breakdown).
• Regulation of the response:
  • Signaling often amplifies the initial signal through cascades (more downstream molecules than initiator).
  • Multiple steps provide control points for specificity and coordination with other pathways.
  • Scaffolding proteins can organize pathway components to boost efficiency and specificity.
  • Termination is essential to reset the system and allow reuse of signaling components.

Signal amplification and specificity

• Amplification: cascades boost the response at each step; persistent active kinases process many substrates before inactivation.
• Specificity and coordination:
  • Different cell types express different receptor, relay, and effector proteins, yielding cell-type-specific responses to the same signal.
  • Pathways can branch or converge; different receptors for the same ligand can produce different outcomes in different cells.
  • Example: liver cell responds to epinephrine with glycogen breakdown; heart cell primarily increases heart rate and contraction.

Key terms to remember

• First messenger: extracellular signal (ligand).
• Second messengers: small, nonprotein molecules/ions (e.g., $cAMP$, $Ca^{2+}$, IP₃, DAG, $cGMP$).
• Kinase vs phosphatase: added phosphate activates kinases; phosphatases remove phosphates to deactivate kinases and terminate signals.
• Phosphorylation cascade: sequential activation of kinases amplifies the signal and creates multiple regulatory checkpoints.
• Termination and regulation: rapid shut-off and control of the signal to prevent aberrant responses.