21.7
Amplificare i segnali tramite secondi messaggeri
Molti ligandi leganti i recettori sono idrofili; non attraversano la membrana cellulare ma si legano ai recettori della superficie cellulare. Pertanto…
I secondi messaggeri sono molecole non proteiche a basso peso molecolare che trasmettono segnali a bersagli all'interno della cellula quando i ligandi si legano ai recettori sulla superficie cellulare.
I secondi messaggeri includono ioni, come gli ioni calcio, nucleotidi ciclici, come l'AMP ciclico, e piccole molecole, come i derivati del fosfatidilinositolo fosfato diacilglicerolo o DAG e l'inositolo trisfosfato o IP3.
Gli ioni calcio sono un secondo messaggero ampiamente diffuso. La concentrazione intracellulare di calcio aumenta in risposta a segnali specifici e innesca diverse funzioni fisiologiche. Ad esempio, nelle cellule muscolari, un aumento del calcio citosolico provoca la contrazione muscolare.
Diverse famiglie di recettori accoppiati a proteine G attivano enzimi che generano secondi messaggeri, tra cui l'adenilil ciclasi che produce AMP ciclico e la fosfolipasi C che produce DAG e IP3.
Questi secondi messaggeri possono attivare più chinasi a valle, amplificando ulteriormente il segnale.
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Q1: What are second messengers and why are they necessary in cell signaling?
Second messengers are low molecular weight non-protein molecules that relay signals from cell surface receptors to targets within the cell. They are necessary because many ligands are hydrophilic and cannot cross the cell membrane, so their message must be relayed by second messengers present in the cytoplasm. Examples include calcium ions, cyclic AMP, and inositol trisphosphate.
Q2: How does the phosphoinositol pathway generate second messengers?
In the phosphoinositol pathway, G protein-coupled receptors activate phospholipase C, which hydrolyzes phosphatidylinositol biphosphate (PIP2) into two second messengers: diacylglycerol (DAG) and inositol triphosphate (IP3). DAG remains near the cell membrane and activates protein kinase C, while IP3 translocates to the endoplasmic reticulum to release calcium ions.
Q3: What is the role of cyclic AMP in the cAMP signaling pathway?
Cyclic AMP (cAMP) is produced by adenylyl cyclase when activated by G protein-coupled receptors. Multiple copies of cAMP are generated from ATP molecules, allowing signal amplification. cAMP can stimulate protein kinase A, open calcium ion channels, and activate the enzyme Exchange-protein activated by cAMP (Epac).
Q4: How do second messengers amplify cellular signals?
Second messengers amplify signals by activating multiple downstream kinases and enzymes. For example, a single activated receptor can trigger adenylyl cyclase to produce many cAMP molecules, and each cAMP molecule can activate protein kinase A, creating an amplifying signals via enzymatic cascade effect that magnifies the original signal.
Q5: What is the function of calcium ions as a second messenger?
Calcium ions are widely occurring second messengers that trigger diverse physiological functions when intracellular concentration increases in response to specific signals. In muscle cells, increased cytosolic calcium results in muscle contraction. Calcium can be released from the endoplasmic reticulum through IP3-activated channels or through other calcium ion channels.
Q6: How does PIP3 differ from other second messengers in growth factor signaling?
Phosphatidylinositol triphosphate (PIP3) is a second messenger derived from phosphorylation of PIP2 when growth factors bind receptor tyrosine kinases. Unlike other second messengers, PIP3 recruits Akt (protein kinase B) to the membrane, where it regulates cell survival pathways including proliferation, apoptosis, and migration.
Q7: What is cyclic GMP and how does it compare to cyclic AMP?
Cyclic GMP (cGMP) is synthesized from GTP when guanylyl cyclase is activated, similar to how cAMP is produced. As a second messenger, cGMP induces protein kinase G (PKG), which has overlapping functions with protein kinase A. However, PKG expression is restricted to vascular tissues, lungs, and the brain, limiting cGMP's physiological scope.