Cell Signaling and Intercellular Communication

Cell Signaling and Intercellular Communication

• Cell-to-cell communication is essential for coordination of cellular activities, maintenance of homeostasis, and control of growth and development.
• There are two types of physiological signals: electrical signals (changes in membrane potential) and chemical signals (secreted by cells into ECF).

Methods of Cell-to-Cell Communication

• Gap junctions: direct cytoplasmic transfer of electrical and chemical signals between adjacent cells (without entering the ECF).
• Contact-dependent signals: occur when surface molecules on one cell membrane bind to surface molecules on another cell’s membrane.
• Local communication: chemicals diffuse through the extracellular fluid (ECF).
• Long-distance communication: combination of electrical signals and chemical signals (hormones).

Intercellular Communication Types

• Neural communication: neurotransmitters released at synaptic junctions from nerve cells and act on a postsynaptic cell.
• Endocrine communication: hormones and growth factors reach cells via the circulating blood or lymph.
• Paracrine communication: products of cells diffuse in the ECF to affect neighboring cells.
• Autocrine communication: binds to receptors on the same cell (cell that secreted the messenger).
• Juxtacrine communication: ligand on one cell surface binds with the receptor on the surface of the next cell (relies on gap junctions or nanotubes).

Signal Transduction

• Signal transduction pathway: a series of steps by which a signal on a cell’s surface is converted into a specific cellular response.
• Signal molecule binds to a receptor, activating the receptor, which in turn activates one or more intracellular signal molecules.
• The last signal molecule in the pathway initiates synthesis of target proteins or modifies existing target proteins to create a response.

Signal Transduction Mechanisms

• Ligand-receptor binding activates the receptor, leading to the activation of intracellular signal molecules.
• Signal pathways can lead to the alteration of transcription of various genes.
• Second messengers bring about many short-term changes in cell function by altering enzyme function, triggering exocytosis, and leading to the alteration of transcription of various genes.

Signal Transduction Pathways

• G protein-coupled receptors: ligand binding opens an ion channel or alters enzyme activity.
• Integrin receptors: ligand binding alters the cytoskeleton.
• Phosphorylation cascade: a series of protein phosphorylations that act as a molecular switch, turning activities on and off.

Signal Amplification

• Signal amplification allows a small amount of signal to have a large effect.
• Receptor-ligand complex activates an amplifier enzyme (AE), which amplifies the signal.

G Protein-Coupled Receptors (GPCR)

• Membrane-spanning proteins with a cytoplasmic tail linked to a G protein.
• When G proteins are activated, they open ion channels in the membrane or alter enzyme activity on the cytoplasmic side of the membrane.

GPCR-Phospholipase C Signal Transduction

• Signal molecule activates receptor and associated G protein.
• G protein activates phospholipase C (PLC), which converts membrane phospholipids into diacylglycerol (DAG) and IP3.
• DAG activates protein kinase C (PKC), which phosphorylates proteins.
• IP3 causes release of Ca2+ from organelles, creating a Ca2+ signal.

Signal Transduction Using Ion Channels

• Receptor-channels open or close in response to signal molecule binding.
• Ion channels allow ions to flow into or out of the cell, creating an electrical signal.