Cell Communication Overview and Mechanisms

Cell Communication Overview

• Cells communicate via signals, which alter cell behavior.
• Unicellular organisms respond to extracellular signals leading to altered cell behavior.
• Multicellular organisms respond to signals by altering metabolism, tissue growth, differentiation, protein synthesis, secretion, intracellular and extracellular composition.
• Signal molecules bind to specific receptors on/in target cells.
• Signal transduction converts extracellular ligand-binding events into intracellular signals that change target cell behaviour.

Signal Molecules & Receptors

• Signal molecules (ligands) can be small (hydrophobic) or large molecules (hydrophilic).
• Different signal molecules elicit diverse responses in cells, based on the receptor and the intracellular machinery.
• Hydrophilic signal molecules bind to cell surface receptors, while hydrophobic signal molecules bind to intracellular receptors.
• Receptors can be intracellular receptors (bind to hydrophobic molecules within the cytoplasm or nucleus) or cell-surface receptors.

Speed of Response

• Speed of cell response to extracellular signals depends on the signal delivery mechanism and the target cell response.
• Rapid responses involve changes in existing proteins (allosteric changes in ion channels, protein phosphorylation).
• Slow responses involve changes in gene expression and new protein synthesis.

Different Types of Cell Responses to Extracellular Signals

• A cell's response to extracellular signals is determined by its receptors and intracellular response mechanisms.
• Cells respond differently to the same extracellular signal molecule depending on their specific receptors and intracellular machinery.
• Signal concentration influences cell responses.

Intracellular Receptors

• Intracellular receptors are located within the cytoplasm or nucleus and bind directly to hydrophobic signal molecules.
• Signal molecules enter the cell and bind to the inactive receptor, causing a change in receptor conformation and activating it.
• Examples of signal molecules that use intracellular receptors include steroids, hormones, and thyroid hormones.

Relaying Signals From Cell Surface Via Second Messengers

• Relaying signals from cell surface involves utilizing second messengers.
• Small molecules (second messengers) carry and distribute intracellular signals.
• Examples include cyclic AMP (cAMP), inositol triphosphate (IP3), and diacylglycerol (DAG).

Intracellular Mediators and Enzymatic Cascades Amplify Extracellular Signals

• Intracellular mediators (second messengers) and enzymatic cascades amplify extracellular signals leading to larger cellular responses.
• Amplification via cascades increases the strength of the signals and affects more downstream processes.
• Feedback mechanisms control the signal strength and regulate the activity of the signal transduction pathways.

Two Important Molecular Switches

• Two important molecular switches are protein phosphorylation and GDP/GTP binding.
• Protein phosphorylation is the addition of phosphate groups to proteins altering their conformation and function.
• GDP/GTP binding regulates protein activity and pathways. GTPase-activating proteins (GAPs) and guanine nucleotide exchange factors (GEFs) contribute to signaling by binding and modulating proteins.

Desensitization of a Signal

• Cells adapt to prolonged or excessive signaling through desensitization mechanisms.
• Negative feedback, delayed feed-forward, receptor inactivation, receptor sequestration, and receptor destruction are various desensitization mechanisms.
• These mechanisms prevent sustained responses to excessive signals.

Three Types of Cell-Surface Receptor Proteins

• Ion channel-coupled, G-protein-coupled, and enzyme-coupled receptors are three types of cell surface receptors.
• Ion channel-coupled receptors mediate fast transmission of signals.
• G-protein-coupled receptors act by indirectly regulating other proteins (enzymes or ion channels).
• Enzyme-coupled receptors either function as enzymes or interact with associated enzymes.

Receptor Tyrosine Kinases (RTKs)

• Ligand binding to RTKs triggers dimerization of receptor molecules, resulting in cross-phosphorylation.
• Cross-phosphorylation activates specific downstream signaling pathways.
• RTKs initiate signaling cascades that change cellular behaviour via protein phosphorylation.

MAP Kinase Signaling Module

• MAP kinase pathways are crucial signaling modules for diverse cellular responses.
• The pathways have three components (MAPKKK, MAPKK, MAPK), with kinases phosphorylating each other to activate the pathway.
• Activated MAPKs can initiate downstream signaling, affecting cell behaviour.

Parallel Intracellular Signaling Pathways

• Multiple intracellular signaling pathways can be activated by either GPCRs or RTKs creating parallel pathways.
• The activated pathways often converge on common signaling components potentially enhancing or opposing other pathways.