Intracellular signaling pathway activation by extracellular signal molecule, which allow cells to respond and adapt to changes in their environment.
The process of activating an intracellular signaling pathway typically involves the following steps:
1) Signal generation:
A signaling molecule (ligand) is produced and released by a signaling cell in response to a specific stimulus.
2) Signal transmission:
The ligand travels through the extracellular space to reach target cells via autocrine, paracrine, endocrine, or juxtracrine signaling.
3) Signal reception:
The target cell detects the ligand through specific receptors, usually located on the cell membrane or within the cell:
G-protein-coupled receptors (GPCRs): Interact with intracellular G proteins upon ligand binding.
Receptor tyrosine kinases (RTKs): Undergo autophosphorylation upon ligand binding, creating docking sites for downstream proteins.
Ligand-gated ion channels: Regulate ion flow across the membrane in response to ligand binding.
Nuclear receptors: Located within the cell and directly regulate gene expression upon ligand binding.
4) Signal transduction:
The receptor undergoes a conformational change upon ligand binding, initiating a cascade of intracellular events.
This often involves the activation or deactivation of proteins, enzymes, and secondary messengers (e.g., cAMP, IP3, DAG, Ca2+), allowing for signal amplification and integration.
5) Cellular response:
The signal transduction pathway leads to changes in the target cell’s activity, such as gene expression, protein synthesis, enzyme activity, or cytoskeletal organization, resulting in outcomes like cell division, differentiation, migration, secretion, or apoptosis.
6) Signal termination:
The cellular response is terminated to maintain sensitivity to new signals.
This is achieved through mechanisms like the removal or degradation of the signaling molecule, receptor inactivation, or deactivation of intracellular signaling proteins.