Cell Signaling and Receptors

• Intracellular signaling proteins act as molecular switches.
• Cell-surface receptors convert chemical signals into electrical signals.
• The main classes of cell-surface receptors are ion-channel-coupled receptors, G-protein-coupled receptors (GPCRs), and enzyme-coupled receptors.
• Bacterial toxins cause disease by altering G protein activity.

Receptor Binding and Activation

• Water-soluble signaling molecules cannot cross the membrane lipid bilayer and instead bind to specific receptors embedded in the plasma membrane.
• Receptors have an extracellular domain for signaling molecule binding and hydrophobic transmembrane and intracellular domains.
• Ligand binding induces a conformational change in the receptor, especially in its intracellular region.
• This conformational change activates a relay of intracellular signaling molecules.
• Receptors are structurally classified into single-pass transmembrane receptors (one extracellular, one transmembrane, one intracellular region) and multipass transmembrane receptors.

Intracellular Signaling Proteins as Molecular Switches

• Many intracellular signaling proteins function as molecular switches.
• Signal receipt causes these proteins to toggle between inactive and active states.
• Once activated, these proteins can stimulate or suppress other proteins in the signaling pathway.
• They remain active until another process switches them off.
• Every activation step has a corresponding inactivation mechanism.
• Activation and inactivation are equally important for a signaling pathway to be useful.
• Proteins acting as molecular switches fall into two classes.
• The largest class consists of proteins activated or inactivated by phosphorylation.
• Intracellular signaling proteins relay, amplify, and modulate signals from cell-surface receptors to intracellular targets.

Molecular Switch Mechanisms

• On/Off Mechanism: Proteins switch "on" to propagate a signal and "off" to terminate it, ensuring signals are tightly regulated.
• Signal Amplification: A single activated switch can trigger multiple downstream molecules, amplifying the signal.

Molecular Switch Classes

• Proteins Regulated by Phosphorylation: Signaling proteins are activated or inactivated by adding or removing a phosphate group.
• Key enzymes involved are:
  • Protein Kinases: Add phosphate groups to specific amino acids (serine, threonine, or tyrosine) in a protein; transfers a phosphate group from an ATP molecule to a protein.
  • Protein Phosphatases: Remove phosphate groups, returning the protein to its inactive state.
  • An example of this mechanism involves Mitogen-activated protein kinases (MAPKs) in cell proliferation pathways.
• GTP-Binding Proteins: Activated when bound to GTP and inactivated when GTP is hydrolyzed to GDP.
• Two main types participate in intracellular signaling:
  • Heterotrimeric G Proteins: Large, trimeric GTP-binding proteins that function in conjunction with G-protein-coupled receptors (GPCRs).
  • Monomeric GTPases: Small GTPases relay signals, such as Ras, which regulates cell growth and differentiation.

Key Enzymes in GTP-Binding Protein Regulation

• Guanine Nucleotide Exchange Factors (GEFs): Promote GDP exchange for GTP, activating the protein.
• GTPase-Activating Proteins (GAPs): Increase the rate of GTP hydrolysis, turning the protein off.

Cell-Surface Receptor Classes

• Cell-surface receptors detect extracellular signals (ligands) and convert them into intracellular responses.
• The three main classes of receptors are:
  • Ion-channel-coupled receptors: Change the plasma membrane permeability to selected ions, altering the membrane potential and producing an electrical current.
  • G-protein-coupled receptors: Activate membrane-bound, trimeric GTP-binding proteins (G proteins), then activates or inhibits an enzyme or ion channel, initiating an intracellular signaling cascade.
  • Enzyme-coupled receptors: Act as or associate with enzymes inside the cell, activating various intracellular signaling pathways when stimulated.

Ion-Channel-Coupled Receptors

• Also known as transmitter-gated ion channels.
• Involved in the direct conversion of chemical signals into electrical signals.
• Ligand binding causes the receptor to open or close an ion channel.
• Alters the flow of ions (Na+, K+, Ca2+) across the plasma membrane, changing the membrane potential.
• Neurotransmitter-gated ion channels in nerve and muscle cells, such as the acetylcholine receptor, are an example.
• Critical for rapid signaling in neurons and muscle contraction.

G-Protein-Coupled Receptors (GPCRs)

• The largest family of cell-surface receptors, with over 700 GPCRs in humans.
• Mediate responses to a wide diversity of extracellular signal molecules, including hormones, local mediators, and neurotransmitters.
• Structure: Seven transmembrane domains coupled to a heterotrimeric G protein (α, β, and γ subunits).
• Composed of three protein subunits (α, β, and γ), two of which are tethered to the plasma membrane by short lipid tails.
• Mechanism: Ligand binding activates the GPCR, causing it to interact with the G protein.
• The G protein exchanges GDP for GTP on its α subunit, activating it.
• The activated G protein regulates target enzymes or ion channels like adenylyl cyclase or phospholipase C.
• Bacterial toxins cause disease by altering G protein activity.
  • Certain toxins disrupt the normal function of G proteins, critical for transmitting signals inside cells.
  • These toxins target the G proteins' ability to regulate intracellular signaling pathways, leading to pathological effects.

Cholera Toxin

• (Produced by Vibrio cholerae).
• Modifies the Gas subunit of stimulatory G proteins by adding an ADP-ribose group (ADP-ribosylation).
• This locks the G protein in its active GTP-bound state.
• Adenylate cyclase is continuously activated, leading to excessive cyclic AMP (cAMP) production.
• High cAMP levels cause intestinal cells to secrete large amounts of water and electrolytes, causing severe diarrhea and dehydration.

Pertussis Toxin

• (Produced by Bordetella pertussis).
• Targets the Gi subunit of inhibitory G proteins.
• Modifies Gai via ADP-ribosylation, preventing it from interacting with receptors and inhibiting its activation.
• This inhibition leads to unregulated cAMP production by adenylate cyclase.
• The dysregulated signaling contributes to whooping cough symptoms, such as excessive mucus production and inflammation.

Disease Impact and Mechanism of Bacterial Toxins

• Mechanism of Action: Both toxins act by chemically modifying G proteins, disrupting normal cycling between active (GTP-bound) and inactive (GDP-bound) states.
• Disease Impact: An imbalance in signaling alters cellular functions, leading to dramatic physiological changes.
• Toxins exploit G protein signaling pathways, which are vital for maintaining cellular and systemic homeostasis.
• By interfering with G proteins, bacterial pathogens disrupt critical processes (fluid balance, immune responses) to promote their survival and spread.

Enzyme-Coupled Receptors

• These receptors either have intrinsic enzymatic activity or associate with enzymes upon activation.
• Typically single-pass transmembrane proteins with an extracellular ligand-binding domain and an intracellular enzyme activity or enzyme-binding domain.
• Their activation mechanism involves a signaling molecule (ligand) binding to the extracellular domain, triggering a conformational change.
• This activates the intracellular enzymatic domain or recruits an associated enzyme to initiate signaling.
• They regulate cell growth, survival, and differentiation.
• Mutations in these receptors can be linked to cancer.

Common Types of Enzyme-Couple Receptors

• Receptor Tyrosine Kinases (RTKs):
  • Most common type of enzyme-coupled receptors.
  • Ligand binding induces dimerization of receptor monomers, activating their tyrosine kinase activity.
  • The activated receptors phosphorylate themselves and other proteins, creating docking sites for intracellular signaling molecules.
• Receptor Serine/Threonine Kinases: Phosphorylate serine or threonine residues on target proteins.
• Cytokine Receptors (associated with Janus Kinases or JAKs): Receptors lacking intrinsic enzymatic activity but recruit cytoplasmic kinases upon activation.

Receptor Signaling Pathways

• Activated receptors often stimulate downstream signaling cascades, including the Ras-MAP kinase pathway.
  • This pathway transduces signals from the extracellular milieu to the cell nucleus, where specific genes are activated for cell growth, division, and differentiation.
• PI3Ks (Phosphoinositide 3-kinases)-Akt pathway.
• These cascades regulate diverse cellular responses, such as metabolism, gene expression.

Receptor Type Differences

• Ion-Channel-Coupled: Open/close ion channels, use electrical (ion) signals, and signals happen quickly (milliseconds), for example, neurotransmitter receptors are of this type.
• GPCRs: Activate G proteins, use chemical (secondary messenger) signals, with an intermediate speed (seconds), for example, adrenaline and serotonin receptors.
• Enzyme-Coupled Receptors: Activate enzymes or kinases, uses protein phosphorylation signals and work slowly (minutes to hours), for example, growth factor receptors.

Conclusion

• Intracellular signaling proteins, as molecular switches, play a central role in ensuring accurate signal transmission by cycling between active and inactive states.
• The three main classes of cell-surface receptors: ion-channel-coupled, GPCRs, and enzyme-coupled receptors enable cells to sense and respond to a variety of extracellular cues.
• Understanding these mechanisms is fundamental for studying cell biology and the basis of many diseases, including cancer and neurological disorders.