Cell Communication and Signalling Pathways

Cell-to-cell communication is essential for multicellular and unicellular organisms, predominantly utilizing chemical messengers.
Local Signalling Mechanisms:
- Cell junctions: Direct cytoplasmic contact via gap junctions in animal cells and plasmodesmata in plant cells.
- Cell-cell recognition: Direct membrane-bound surface protein interactions between adjacent cells.
- Paracrine signalling: Secretion of local regulators (such as growth factors) affecting nearby target cells.
- Synaptic signalling: Release of neurotransmitters across a synapse triggered by an electrical signal.
Long-Distance Signalling:
- Endocrine (hormonal) signalling: Release of hormones into body fluids (such as the bloodstream) to reach distant target cells bearing specific receptors.
Three Stages of Cell Signalling:
- Reception: Binding of an extracellular signalling molecule (ligand) to a specific target cell receptor.
- Transduction: A conformational change in the receptor initiates a signal transduction pathway, often consisting of a multistep cascade.
- Response: The transduced signal triggers a specific cellular activity (such as gene transcription or enzymatic catalysis).

Ligand binding is highly specific and induces a conformational change in the receptor, which initiates transduction.
G Protein-Coupled Receptors (GPCRs):
- Largest family of cell-surface receptors, characterized by a polypeptide spanning the plasma membrane with seven transmembrane helices.
- Works with a G protein acting as an on/off switch: bound to GDP when inactive, and bound to GTP when active.
- Ligand binding causes the GPCR to activate the G protein by exchanging GDP for GTP; the active G protein then activates a downstream enzyme.
- The G protein inactivates itself by hydrolyzing bound GTP to GDP and inorganic phosphate ( $\text{P}_i$ ).

Receptor Tyrosine Kinases (RTKs):
- Exist as individual monomers with an extracellular ligand-binding domain, a membrane-spanning $\alpha$ helix, and an intracellular tail with multiple tyrosines.
- Ligand binding causes two receptor monomers to form a dimer.
- Dimerization activates tyrosine kinase activity, leading to autophosphorylation of tyrosines using ATP ( $6\text{ ATP} \rightarrow 6\text{ ADP}$ ).
- Specific relay proteins bind the phosphorylated tyrosines, triggering multiple transduction pathways simultaneously.

Ligand-Gated Ion Channel Receptors:
- Remain closed until a specific signalling molecule binds to the receptor.
- Channel gate opens upon ligand binding, allowing the rapid flow of specific ions (e.g., $\text{Na}^+$ , $\text{Ca}^{2+}$ ) down their concentration gradient.
- Dissociation of the ligand causes the gate to close.

Located in the cytosol or nucleus of target cells rather than in the plasma membrane.
Activated by small or hydrophobic signalling molecules that cross the lipid bilayer directly (e.g., steroid and thyroid hormones, such as aldosterone).
The resulting active hormone-receptor complex translocates into the nucleus, functioning directly as a transcription factor to turn on specific genes.

Multistep pathways allow both amplification of the signal (a few molecules yield a massive response) and tighter regulation/coordination.
Protein Phosphorylation and Dephosphorylation:
- Protein kinases: Transfer phosphate groups from ATP to target proteins (phosphorylation), usually activating them in a phosphorylation cascade.
- Protein phosphatases (PP): Catalyze the removal of phosphate groups (dephosphorylation), inactivating proteins and turning the pathway off.
Second Messengers:
- Small, nonprotein, water-soluble molecules or ions that spread throughout the cytosol by rapid diffusion.
- Cyclic AMP (cAMP):
- Adenylyl cyclase in the plasma membrane converts ATP to cAMP in response to an extracellular signal.
- Phosphodiesterase hydrolyzes cAMP into AMP to terminate the signal.
- cAMP typically activates protein kinase A (PKA), which phosphorylates downstream target proteins.
- Calcium Ions ( $\text{Ca}^{2+}$ ):
- Cytosolic concentration of $\text{Ca}^{2+}$ is maintained at very low levels compared to extracellular fluid and the endoplasmic reticulum (ER) lumen via ATP-driven pumps.
- Pathways releasing $\text{Ca}^{2+}$ utilize phospholipase C to cleave the plasma membrane phospholipid $\text{PIP}_2$ into diacylglycerol (DAG) and inositol trisphosphate ( $\text{IP}_3$ ).
- $\text{IP}_3$ diffuses through the cytosol to open $\text{IP}_3$ -gated calcium channels in the ER membrane, elevating cytosolic $\text{Ca}^{2+}$ to activate downstream proteins.

Nuclear Responses: Involve transcription factors regulating gene expression to synthesize specific mRNA and proteins.
Cytoplasmic Responses: Involve direct regulation of enzymatic activities (e.g., epinephrine activating glycogen phosphorylase to break down glycogen into $10^8\text{ molecules}$ of glucose 1-phosphate).
Specificity and Coordination:
- Different cell types produce distinct suites of proteins, allowing the same signalling molecule to elicit different responses in different cells.
- Signal coordination features include single linear pathways, branched pathways triggering multiple responses, cross-talk between convergent pathways, and alternative receptors producing distinct outputs.

Programmed cell death that degrades damaged, infected, or aged cells, preventing damaging enzymes from leaking into neighboring tissue.
Cellular components are systematically disassembled and packaged into membrane blebs, which are engulfed and digested by scavenger cells.
Molecular Mechanism in Caenorhabditis elegans:
- In the absence of a death signal, active Ced-9 located on the outer mitochondrial membrane continuously inhibits Ced-4.
- Binding of a death-signalling ligand inactivates Ced-9, relieving inhibition of Ced-4.
- Ced-4 activates the primary caspase protease Ced-3, triggering a cascade that activates nucleases and other proteases to dismantle the cell.

Triggers and Roles:
- Triggered by extracellular death signals, nuclear DNA damage, or ER protein misfolding.
- Essential for embryonic morphogenesis, including digit development via apoptosis of interdigital webbing in mammals.
- Improper apoptosis regulation contributes to neurodegenerative diseases (e.g., Alzheimer's, Parkinson's) and cancers.