Signal Transduction Lecture Notes

Signal Transduction Lecture Notes

• Signal transduction is the conversion of information from one form into another
• It begins when a target cell receives an incoming extracellular (EC) signal and converts it into intracellular (IC) signals that alter cell behavior
• Cell-cell communication is crucial for coordinating the behavior of unicellular and multicellular organisms. This includes growth, development, differentiation, tissue and organ formation, maintenance, homeostasis, and disease processes.
• An example of cell signaling is the fight-or-flight response which is triggered by a signaling molecule called epinephrine.
• The process of signal transduction involves three stages:
  • Reception: The signaling molecule (ligand) binds to a specific receptor protein on the target cell. This binding initiates a series of events within the cell.
  • Transduction: The binding of the signaling molecule to the receptor protein triggers a cascade of intracellular signaling molecules. A relay team of communication proteins in the cell are activated.
  • Response: The final signal triggers a specific cellular response, such as activating a gene, triggering apoptosis, entering mitosis, or inducing secretion.
• Signal transduction involves the conversion of extracellular signals to intracellular signals.
• Cell communication is similar to human communication but cells have their own special messages.
• There are three types of cell-surface receptors : ion-channel-coupled, G-protein-coupled (GPCRs), and enzyme-coupled receptors .
• GPCRs activate G-proteins, which then transmit signals and turn off.

Signal molecule classification

• Hydrophilic/large signal molecules rely on cell-surface receptors. Examples include growth factors and some proteins that relay signals in the central nervous system.
• Hydrophobic/small signal molecules activate intracellular receptors. Steroid hormones are an example of this type of signal.

Important signal molecules

• Hormones: Examples include epinephrine (adrenaline), cortisol, estradiol, insulin, testosterone, and thyroid hormone. Various tissues produce hormones, and hormones transmit messages throughout the body. Specific hormones affect particular cells. The effects of hormones can vary based on the target cells in their specific responses.
• Local Mediators: Other signal molecules, such as Epidermal growth factor (EGF), Platelet-derived growth factor (PDGF), Nerve growth factor (NGF), Histamine, Nitric oxide, Acetylcholine and γ-Aminobutyric acid (GABA), regulate cellular interactions in the body locally. They target nearby cells.

Signal transduction and disease

• Defective signaling pathways lead to diseases.
• Drugs, poisons, and pesticides often target signaling pathways to modify cellular activity.
• For example, cholera toxin affects G-proteins, blocking their ability to hydrolyze GTP. This results in prolonged activation of the G protein, abnormal chloride and water outflow into the gut, causing diarrhea, and dehydration.

Quiz Questions

• Which signaling molecule is an example of a hormone that acts in endocrine signaling?
  • Insulin
• Cholera toxin interferes with which part of the G-protein process?
  • Prevents Gs from hydrolyzing GTP

Lecture Outline of Signal Transduction

• Definitions
• General principles of cell signaling and Signal Transduction
• Types of intercellular signaling
• Chemical nature of some signal molecules
• Different responses of cells to the same signal molecule
• Molecular on/off switching in signal transduction
• Signal transduction through the various types of receptors