Signal Transduction Overview

Signal Transduction Homework Notes

Types of Primary Signaling Receptors:
1. G-Protein Coupled Receptors (GPCRs)
- Work by binding a ligand, causing a conformational change that activates intracellular G-proteins. These proteins then trigger various signaling cascades.
1. Tyrosine Kinase Receptors (TRKs)
- Upon ligand binding, these receptors autophosphorylate on tyrosine residues and activate downstream signaling pathways.
1. Ion Channel Receptors
- Open or close in response to ligand binding, altering the flow of ions and creating a change in membrane potential, which transduces a signal.
1. Nuclear Receptors
- Ligands pass through the membrane to bind to receptors in the nucleus, regulating gene transcription directly.
1. Receptor Serine/Threonine Kinases
- Activate through ligand binding, phosphorylating serine and threonine residues to initiate downstream signaling.

List of Targets for GPCRs:
1. Adenylyl Cyclase
- Converts ATP to cyclic AMP (cAMP), a second messenger that mediates various physiological responses.
1. Phospholipase C (PLC)
- Produces inositol trisphosphate (IP3) and diacylglycerol (DAG) from phosphatidylinositol, triggering calcium release and protein kinase C activation.
1. Ion Channels
- Open or close in response to GPCR activation, influencing ion concentrations across the membrane.
1. Phosphodiesterases (PDEs)
- Degrade cyclic AMP or cyclic GMP to regulate their levels in the cell.
1. Rho Kinase (ROCK)
- Activates signaling pathways involved in cytoskeletal reorganization.

Function of TRKs:
- These receptors act by autophosphorylation upon activation, which propagates the signal by activating various downstream effectors.
Organisms with TRKs:
- Found in multicellular organisms, including animals and plants.
Secondary Signaling Pathways Activated by TRKs:
1. MAPK Pathway (Mitogen-Activated Protein Kinase Pathway)
2. PI3K/Akt Pathway

Examples of Signaling Systems:
1. G-Protein Coupled Receptor Signaling
- Involves cAMP-mediated signaling through protein kinase A (PKA).
1. Insulin Signaling Pathway
- Triggered by insulin binding to TRK leading to activation of Akt involved in glucose metabolism.
1. Growth Factor Signaling through TRKs
- Activation leads to proliferation and differentiation.

Protein Kinases:
- Enzymes that catalyze the transfer of a phosphate group from ATP to specific amino acids in target proteins (e.g., serine, threonine, or tyrosine).
- Example:
- Protein Kinase A (PKA) activates various target proteins via phosphorylation to modulate their activity.
Protein Phosphatases:
- Enzymes that remove phosphate groups from proteins, reversing the action of kinases.
- Example:
- Protein Phosphatase 1 (PP1) is involved in dephosphorylating glycogen phosphorylase, thereby regulating glucose metabolism.

Can Ion Channels be Signal Receptors?
- Yes.
Examples of Ion Channels:
1. Ligand-Gated Ion Channels
- Respond to the binding of a neurotransmitter which causes channel opening (e.g., nicotinic acetylcholine receptor allowing Na+ influx).
1. Voltage-Gated Ion Channels
- Open in response to changes in membrane potential, leading to action potential generation (e.g., sodium channels in neurons).
Intracellular Transmission:
- Ion flow (e.g., Ca2+) changes the intracellular environment, affecting further signaling pathways.

Three Most Important Second Messengers:
1. Cyclic AMP (cAMP)
- Mediates the effects of hormones like adrenaline; activates PKA.
1. Inositol Trisphosphate (IP3)
- Triggers release of calcium from the endoplasmic reticulum, crucial for various signaling pathways.
1. Calcium Ions (Ca2+)
- Acts as a universal messenger in muscle contraction, neurotransmitter release, and other cellular responses.

Function:
- Converts ATP into cAMP, serving as a second messenger in various signaling pathways to activate downstream processes through protein kinases.

Cyclic AMP (cAMP):
- Derived from ATP; functions primarily in regulating energy balance through PKA.
- Plays critical roles in neurotransmission and hormonal responses.
Cyclic GMP (cGMP):
- Derived from GTP; primarily involved in signaling mechanisms related to vasodilation and smooth muscle relaxation.
- Activates protein kinase G (PKG).

Examples:
1. Wnt/β-catenin Pathway
- Involved in developmental processes; β-catenin translocates to the nucleus to regulate gene expression.
1. Notch Signaling Pathway
- Mediates cell fate determination; Notch intracellular domain acts as a transcription factor.
1. Hedgehog Signaling Pathway
- Involves downstream regulation of transcription factors in embryonic development.

Examples:
1. GPCR and Phospholipase C
- Calcium signaling through IP3 and DAG without direct gene regulation.
1. Cyclic AMP signaling through PKA
- Modulates existing enzymes rather than changing gene expression directly.

Two Pathways Regulating Calcium Signaling:
1. Phospholipase C Pathway
- Produces IP3, which promotes calcium release from the endoplasmic reticulum.
1. Ryanodine Receptor Pathway in Muscle Cells
- Calcium-induced calcium release mechanism that enhances muscle contraction.

Mechanism:
Upon ligand binding (e.g., neurotransmitter), these channels open to allow Ca2+ influx.
This influx can trigger signaling cascades, including activating enzymes or modifying gene expression, contributing to various cellular functions.

Example:
- Different hormones (e.g., adrenaline vs insulin) can modify how cells respond to calcium signaling despite them all utilizing similar pathways.
Explanation:
- Cells exhibit different receptor types and levels of second messenger which dictate distinct biological responses even when receiving similar initial signals.