Cellular Signaling and cAMP Regulation

Cell-Cell Communication

• Cells need to communicate to proliferate, differentiate, migrate, and maintain a functional state.
• Cell-cell signaling coordinates cellular function within and between tissues, up to the organism level.
• Signaling informs cells about their identity, location, and assigned roles.
• External signals are converted into internal responses. Prokaryotes respond mostly to chemical signals, while humans respond to various cues like light, sound, and chemicals in food.
• Communication occurs via direct contact or chemical signals.

Local and Long-Distance Signaling

• Multicellular organisms use chemical messengers for communication.
• Animal and plant cells communicate directly via cell junctions.
• In local signaling, animal cells may communicate via direct contact.
• Animal cells also use local regulators (messenger molecules) for short-distance signaling.
• For long-distance signaling, plants and animals use hormones.

Signals act over different ranges

• Endocrine signaling uses hormones to travel through the bloodstream to distant target cells (e.g., estrogen, epinephrine).
• Paracrine signaling involves local regulators acting on nearby cells (e.g., nitric oxide, histamine, prostaglandins).
• Neuronal/synaptic signaling involves neurotransmitters released across synapses to target cells (e.g., neurotransmitters).

Signal Transduction Pathway

• Extracellular signals are received by receptors on the cell surface.
• Intracellular signaling pathways convert the signal into a cellular response.
• A signal receptor (sensor), transduction cascade, targets, and the overall response are crucial parts of the pathway.
• The response can change metabolic activity, gene expression, and cytoskeletal protein function, influencing cell shape or motility.

Signal

• Signal = ligand; a molecule that binds to a specific receptor site on a protein.
• Ligands include peptides/proteins (growth factors), amino acid derivatives (e.g., epinephrine, histamine), small biomolecules (ATP, steroids, prostaglandins), gases (nitric oxide), photons, and damaged DNA.

Receptors

• Receptors sense and initiate signal transduction.
• Extracellular receptors (cell surface receptors) are common in enzyme-linked receptors, exemplified by growth factor receptors.
• Intracellular receptors (steroid receptors) reside inside the cell, typically for small/hydrophobic chemical transmitters.

Intracellular Receptors

• Some receptor proteins are intracellular, situated in the cytosol or nucleus of target cells.
• Small or hydrophobic chemical messengers easily cross the cell membrane and activate receptors.
• Steroid and thyroid hormones are examples of hydrophobic messengers.
• Activated hormone-receptor complexes act as transcription factors, modifying gene expression.

3 Types of Extracellular Cell-surface Receptors

• Ion channel-linked receptors act as gates, allowing specific ions (e.g., Na+, Ca2+) to pass when a ligand binds.
• Trimeric G protein-linked receptors act as on/off switches. GDP binding inactivates the G protein. GTP binding activates it.
• Enzyme-linked receptors (tyrosine kinase) are membrane-bound receptors that phosphorylate target molecules upon ligand binding.

Trimeric G protein-linked receptors

• A G protein-coupled receptor (GPCR) is a type of plasma membrane receptor that works with the help of a G protein.
• The G protein acts as an on/off switch. If GDP is bound to the G protein, the G protein is inactive.

G-protein activation

• Ligand binding triggers GDP-GTP exchange, causing the G protein to become active.
• The alpha subunit dissociates from the beta-gamma complex.
• The active G protein subunits alter the activity of effector enzymes.

All G proteins are similar in structure/activation.

• Two primary classes of pathways:
  • Adenylyl cyclase pathway
  • Phospholipase C pathway

An Activated G\o-Protein-GTP

• An activated G protein activates adenylyl cyclase to produce cyclic AMP (cAMP), a second messenger.
• cAMP activates protein kinase A (PKA).
• Various cellular responses are ultimately initiated.

Protein Kinase A

• Protein kinase A (PKA) phosphorylates downstream target enzymes, typically by adding a phosphate group to them.
• This phosphorylation typically alters enzyme activity, leading to the desired cellular response.

A Signal Cascade (Amplification)

• Binding of epinephrine to a receptor sets off a cascade of events.
• Each step causes a significant increase in number of molecules, leading to a amplified response.

Protein Phosphorylation and Dephosphorylation

• Many pathways for signal transmission involve a cascade of protein phosphorylations.
• Protein phosphatases remove phosphate groups to deactivate enzymes, allowing a switch in activity.

What are targets for Protein Kinase A?

• Various pathways regulated by cAMP.
• Glycogen breakdown: Targets include muscle, liver.
• Heart rate: Targets are cardiovascular, kidney.
• Water reabsorption: Targets are kidney.

Target protein Phospholipase C

• The target effector enzyme in this pathway is phospholipase C (PLC).
• PLC cleaves the membrane phospholipid, phosphatidylinositol (PIP\2) into two second messengers: inositol-1, 4, 5-trisphosphate (InsP3), and diacylglycerol (DAG).

Calcium Signaling

• DAG activates protein kinase C, which alters cellular responses.
• InsP3 acts as a second messenger to trigger the release of Ca2+ from intracellular stores.
• Calcium ions participate in various cellular events, such as smooth muscle contraction

Receptor Tyrosine Kinases

• Receptor tyrosine kinases (RTKs) are cell surface receptors.
• Upon ligand binding, RTKs dimerize and cross-phosphorylate their cytoplasmic tyrosine residues.
• Phosphorylated tyrosines serve as docking sites for signaling proteins, leading to cellular responses.
• An example is the insulin receptor.

Receptors for everything

• Receptors fit into specific shapes (lock-and-key model), a key feature of receptor specificity.

Proto-oncogenes- Signaling Proteins

• Proto-oncogenes are proteins that promote cell transformation.
• Many oncogenes are viral protein kinases that are constitutively active.
• Examples include Serine-Threonine (like c-raf, ERK, akt) and Tyr kinases (like src, abl).

Activation of Epidermal Growth Factor Receptor by Mutation

• Some receptors can activate even in the absence of a ligand, due to mutation.
• The v-erbB of AEV lacks the C-terminal domains necessary for regulation.
• Such mutations play a role in human breast cancer.

Cancer Drug-targeting therapy

• The strategy targets areas inside the signaling pathway to block the spread of cancer cells.

Termination of the Signal

• Inactivation mechanisms are crucial for controlling and terminating signaling pathways.
• When signal molecules leave the receptor, the receptor returns to an inactive state.

Intracellular Signaling & Diseases

• Cholera is an example of a disruption of a signal transduction pathway.
• Vibrio cholerae produces an enterotoxin that modifies Gs, preventing proper deactivation of the system, leading to severe fluid loss.

Beta Blockers

• Some drugs have similar shapes to neurotransmitters, enabling them to bind to receptors and block their activation.

Ion channel receptors

• Ion channel receptors act as gates in the membrane; shape changes trigger opening.
• Specific ions pass through the channel.
• Examples: muscle contraction, nerve cell communication.
  • The Na+/K+ ATPase is crucial for maintaining the membrane potential.

Summary

• Cell signaling mechanisms (endocrine, paracrine, synaptic, and cell-cell contact).
• Signal transduction pathways, receptor proteins
• Example events: ligand-gated ion channels (e.g., Na+ influx), 7-transmembrane receptors (e.g., G protein activation), and receptor tyrosine kinases.
• Second messengers (cAMP, DAG, IP3, Ca2+), and their roles in activating various enzymes and signal cascades.
• Amplification of signaling events.
• Inactivation mechanisms (i.e., removal of ligand, hydrolysis of GTP, activity of phosphatases and pumps).

This quiz explores key concepts related to cellular signaling pathways, focusing on the role of cAMP and protein phosphorylation cascades. Test your understanding of the second messengers involved and the targets of Protein Kinase A. Perfect for students studying cell biology or biochemistry.