Cell Communication

1. Cell Communication

2. Overview: The Cellular Internet • Cell-to-cell communication is absolutely essential for multicellular organisms • Nerve cells must communicate pain signals to muscle cells (stimulus) in order for muscle cells to initiate a response to pain

3. Biologists have discovered some universal mechanisms of cellular regulation

4. External Signals Signal Transduction Pathway

5. Exchange of mating factors. Each cell type secretes a mating factor that binds to receptors on the other cell type. factor 3 1 2 Receptor a  factor Yeast cell, mating type  Yeast cell, mating type a Mating. Binding of the factors to     receptors induces changes      in the cells that     lead to their     fusion.  a New a/ cell. The nucleus of the fused cell includes all the genes from the a and a cells. a/ • Yeast cells identify their mates by cell signaling (early evidence of signaling)

6. Signal Transduction Pathways • Convert signalson a cell’s surface into cellular responses • Are similar in microbes and mammals, suggesting an early origin

7. Local and Long-Distance Signaling • Cells in a multicellular organism (tissues, organs, systems) communicate via chemical messengers • A hormone is a chemical released by a cell in one part of the body, that sends out messages that affect cells in other parts of the organism • All multicellular organisms produce hormones • Plant hormones are also called phytohormones • Hormones in animals are often transported in the blood

8. Plasma membranes Plasmodesmata between plant cells Gap junctions between animal cells Figure 11.3 (a) Cell junctions. Both animals and plants have cell junctions that allow molecules to pass readily between adjacent cells without crossing plasma membranes. • Animal and plant cells • Have cell junctions that directly connect the cytoplasm of adjacent cells

9. Figure 11.3 (b) Cell-cell recognition. Two cells in an animal may communicate by interaction between molecules protruding from their surfaces. • In local signaling, animal cells • May communicate via direct contact

10. Local signaling Target cell Electrical signal along nerve cell triggers release of neurotransmitter Neurotransmitter diffuses acrosssynapse Secretory vesicle Local regulator diffuses through extracellular fluid Target cell is stimulated (b) Synaptic signaling. A nerve cell releases neurotransmitter molecules into a synapse, stimulating the target cell. (a) Paracrine signaling. A secreting cell acts on nearby target cells by discharging molecules of a local regulator (a growth factor, for example) into the extracellular fluid. • In other cases, animal cells • Communicate using local regulators

11. Long-distance signaling Blood vessel Endocrine cell Hormone travels in bloodstream to target cells Target cell (c) Hormonal signaling. Specialized endocrine cells secrete hormones into body fluids, often the blood. Hormones may reach virtually all body cells. Figure 11.4 C • In long-distance signaling • Both plants and animals use hormones (e.g. Insulin)

12. The Three Stages of Cell Signaling • Earl W. Sutherland • Discovered how the hormone epinephrine acts on cells

13. Sutherland’s Three Steps • Sutherland suggested that cells receiving signals went through three processes • Reception • Transduction • Response

14. EXTRACELLULAR FLUID CYTOPLASM Plasma membrane 1 2 3 Reception Transduction Response Receptor Activation of cellular response Relay molecules in a signal transduction pathway Signal molecule Figure 11.5 • Overview of cell signaling

15. Step One - Reception • Reception occurs when a signal molecule binds to a receptor protein, causing it to change shape • Receptor protein is on the cell surface

16. The binding between signal molecule (ligand) and receptor is highly specific • A conformational change in a receptor is often the initial transduction of the signal

17. Step Two - Transduction • The binding of the signal molecule alters the receptor protein in some way • The signal usually starts a cascade of reactions known as a signal transduction pathway • Multistep pathways can amplify a signal

18. Step Three - Response • Cell signaling leads to regulation of cytoplasmic activities or transcription • Signaling pathways regulate a variety of cellular activities

19. Hormone (testosterone) EXTRACELLULAR FLUID 1 The steroid hormone testosterone passes through the plasma membrane. Plasma membrane Receptor protein 2 Testosterone binds to a receptor protein in the cytoplasm, activating it. Hormone- receptor complex 3 The hormone- receptor complex enters the nucleus and binds to specific genes. DNA mRNA 4 The bound protein stimulates the transcription of the gene into mRNA. NUCLEUS New protein 5 The mRNA is translated into a specific protein. CYTOPLASM Figure 11.6 Example of Pathway • Steroid hormones bind to intracellular receptors

20. Growth factor Reception Receptor Phosphorylation cascade Transduction CYTOPLASM Inactive transcription factor Active transcription factor Response P DNA Gene Figure 11.14 mRNA NUCLEUS • Other pathways regulate genes by activating transcription factors that turn genes on or off

21. Termination of the Signal • Signal response is terminated quickly by the reversal of ligand binding

22. Receptors in the Plasma Membrane • There are three main types of membrane receptors: • G-protein-linked • Tyrosine kinases • Ion channel

23. Signal-binding site Figure 11.7 Segment that interacts with G proteins Inactivate enzyme ActivatedReceptor G-protein-linked Receptor Signal molecule Plasma Membrane GDP G-protein(inactive) GTP GDP CYTOPLASM Enzyme Activated enzyme GTP GDP Pi Cellular response • G-protein-linked receptors

24. Signal-binding site Signalmolecule Signal molecule Helix in the Membrane Tyr Tyr Tyr Tyr Tyrosines Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Receptor tyrosinekinase proteins(inactive monomers) Dimer CYTOPLASM Figure 11.7 Activatedrelay proteins Cellularresponse 1 Tyr Tyr Tyr Tyr Tyr Tyr P P Tyr P Tyr Tyr Tyr Tyr P Tyr Tyr Tyr P P P Tyr Tyr Tyr Tyr Tyr P Tyr Tyr Tyr Cellularresponse 2 P P P Tyr Tyr P 6 ATP 6 ADP Activated tyrosine- kinase regions (unphosphorylated dimer) Fully activated receptor tyrosine-kinase (phosphorylated dimer) Inactiverelay proteins • Receptor tyrosine kinases

25. Gate closed Signalmolecule(ligand) Ions Ligand-gated ion channel receptor Plasma Membrane Gate open Cellularresponse Gate close Figure 11.7 • Ion channel receptors