Cell Signalling Summary

Autocrine Signaling: A cell targets itself or similar adjacent cells by releasing chemical mediators (autacoids) locally without entering systemic circulation (e.g., cytokines such as interferons, interleukins, and lymphokines).
Paracrine Signaling: Non-nervous cells release chemical mediators that act locally on nearby target cells over short distances to produce rapid effects (e.g., histamine and prostaglandins).
Gap Junction Signaling: Signaling occurs directly between cells connected via gap junctions.
Endocrine Signaling: Endocrine cells secrete hormones into the bloodstream to reach distant target cells possessing specific cognate receptors.

G-Protein-Coupled Receptors (GPCRs):
- Polypeptide or hydrophilic hormones bind to cell-surface receptors, inducing conformational changes and protein-protein interactions.
- Receptor activation triggers the exchange of guanosine diphosphate ( $GDP$ ) for guanosine triphosphate ( $GTP$ ) on the inactive $\alpha$ , $\beta$ , and $\gamma$ G-protein complex.
- The $GTP$ -bound $\alpha$ subunit dissociates from the $\beta$ and $\gamma$ subunits to interact with membrane-bound target proteins and enzymes, initiating downstream intracellular signaling cascades.

Enzyme-Linked Receptors:
- Receptors exist as homodimers that function directly as enzymes or activate associated intracellular enzymes upon ligand binding.
- Example: Leptin binds to the extracellular region of its receptor, causing phosphorylation and activation of intracellular Janus Kinase 2 ( $JAK2$ ).
- Activated $JAK2$ phosphorylates signal transducer and activator of transcription ( $STAT$ ) proteins, which enter the nucleus to activate target gene transcription and protein synthesis.

Lipid-soluble steroid hormones diffuse across the cell membrane into the cytoplasm or nucleus.
The hormone binds to an intracellular nuclear receptor to form a hormone-receptor complex.
The complex binds directly to the hormone response element (promoter region) on $DNA$ to activate or inhibit gene transcription and subsequent protein expression.