Cell Communication

Communication Methods Cell-to-cell contact Local signaling Long distance signaling

Cell-to-Cell Communications Cell junctions directly connect the cytoplasm of adjacent cells Ex: cardiac cells for rhythmicity; plamodesmata between plant cells Surface receptors can give/send information Ex: specific immune response VIDEO Plasma membranes Plasmodesmata between plant cells Gap junctions between animal cells

diffuses across synapse Local Signaling VIDEO Adjacent cells are signaled. Chemical messengers released Ex: Neurotransmitters via neurons (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. (b) Synaptic signaling. A nerve cell releases neurotransmitter molecules into a synapse, stimulating the target cell. Local regulator diffuses through extracellular fluid Target cell Secretory vesicle Electrical signal along nerve cell triggers release of neurotransmitter Neurotransmitter diffuses across synapse is stimulated Local signaling

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

Long Distance Signaling Use of hormones Both plants and animals use hormones (e.g. Insulin) Can affect many cells in Other parts of the body Protein or Steroid types Hormone travels in bloodstream to target cells (c) Hormonal signaling. Specialized endocrine cells secrete hormones into body fluids, often the blood. Hormones may reach virtually all body cells. Long-distance signaling Blood vessel Target cell Endocrine cell Figure 11.4 C

How Does it Work? Signal Transduction Pathways Convert signals on a cell’s surface into cellular responses Are similar in microbes and mammals, suggesting an early origin

Relay molecules in a signal transduction pathway 3 Phases of Signal Transduction EXTRACELLULAR FLUID Receptor Signal molecule Relay molecules in a signal transduction pathway Plasma membrane CYTOPLASM Activation of cellular response Figure 11.5 Reception 1 Transduction 2 Response 3

Step One - Reception Reception occurs when a signal molecule (ligand) binds to a receptor protein. Ligand and receptor have a unique bonding

Step Two - Transduction Signal initiated by conformational change of receptor protein Signal is turned into a cellular response. Signaling cascades relay signals to target Multistep pathways can amplify a signal Second messengers involved

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

A phosphorylation cascade Signal molecule Active protein kinase 1 2 3 Inactive protein kinase Cellular response Receptor P ATP ADP PP Activated relay molecule i Phosphorylation cascade P  A relay molecule activates protein kinase 1. 1 2 Active protein kinase 1 transfers a phosphate from ATP to an inactive molecule of protein kinase 2, thus activating this second kinase. Active protein kinase 2 then catalyzes the phos- phorylation (and activation) of protein kinase 3. 3 Enzymes called protein phosphatases (PP) catalyze the removal of the phosphate groups from the proteins, making them inactive and available for reuse. 5 Finally, active protein kinase 3 phosphorylates a protein (pink) that brings about the cell’s response to the signal. 4 Figure 11.8

Benefits of a 2° messenger system 1 signal Activated adenylyl cyclase receptor protein 2 Not yet activated amplification 4 amplification 3 cAMP amplification 5 GTP G protein protein kinase 6 amplification Amplification! enzyme Cascade multiplier! 7 amplification VIDEO FAST response! product

Pathways can also regulate genes by activating transcription factors that turn genes on or off Reception Transduction Response mRNA NUCLEUS Gene P Active transcription factor Inactive DNA Phosphorylation cascade CYTOPLASM Receptor Growth factor Figure 11.14

Types of Receptors There are three main types of plasma membrane receptors: G-protein-linked Tyrosine kinases Ion channel

G-protein-linked receptors Very common Results in a single pathway response G-protein-linked Receptor Plasma Membrane Enzyme G-protein (inactive) CYTOPLASM Cellular response Activated enzyme Activated Receptor Signal molecule Inactivate GDP GTP P i

Receptor tyrosine kinases Multiple pathway response Signal molecule Signal-binding site CYTOPLASM Tyrosines Signal molecule Helix in the Membrane Tyr Dimer Receptor tyrosine kinase proteins (inactive monomers) P Cellular response 1 Inactive relay proteins Activated relay proteins Cellular response 2 Activated tyrosine- kinase regions (unphosphorylated dimer) Fully activated receptor tyrosine-kinase (phosphorylated 6 ATP 6 ADP Figure 11.7

Ion channel receptors When ligand binds, channel can open or close. Cellular response Gate open Gate close Ligand-gated ion channel receptor Plasma Membrane Signal molecule (ligand) Figure 11.7 Gate closed Ions When ligand binds, channel can open or close. Ex: neurotransmitters bind as ligands for Na+ ion channels

*Intracellular Receptors Target protein is INSIDE the cell Must be hydrophobic molecule Hormone (testosterone) EXTRACELLULAR FLUID Receptor protein DNA mRNA NUCLEUS CYTOPLASM Plasma membrane Hormone- receptor complex New protein Figure 11.6 1 The steroid hormone testosterone passes through the plasma membrane. Why can the signal molecule meet its target INSIDE the cell? Testosterone binds to a receptor protein in the cytoplasm, activating it. 2 The hormone- receptor complex enters the nucleus and binds to specific genes. 3 The bound protein stimulates the transcription of the gene into mRNA. 4 The mRNA is translated into a specific protein. 5

Evolutionary Significance Unicellular and multicellular cell communication have similarities Yeast cells signal for sexual reproduction through signal transduction process. Bacteria secrete molecules to sense density of own population. Quorum Sensing (survival purpose) TEDED on Quorum Sensing