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Cell Communication
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The “Cellular Internet”
Biologists have discovered some universal mechanisms of cellular regulation that involve cell-to-cell communication. External signals are converted into responses within the cell
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Methods used by Cells to Communicate
Cell Signaling using chemical messengers 4 types of cell signaling Juxtacrine – direct contact between cells Paracrine – diffuse to nearby cells (neurons) Autocrine – effects the same cells Endocrine – Hormones travel to distant cells
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Cell-Cell Communication
Animal and plant cells Have cell junctions that directly connect the cytoplasm of adjacent cells 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.
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Cell-Cell Communication
Animal cells use gap junctions to send signals Cells must be in direct contact Protein channels connecting two adjoining cells Gap junctions between animal cells
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Cell-Cell Communication
Plant cells use plasmodesmata to send signals Cells must be in direct contact Gaps in the cell wall connecting the two adjoining cells together Plasmodesmata between plant cells
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Cell-Cell Recognition
In local signaling, animal cells may communicate via direct contact Membrane bound cell surface molecules Glycoproteins Glyolipids Figure 11.3 (b) Cell-cell recognition. Two cells in an animal may communicate by interaction between molecules protruding from their surfaces.
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Local Signaling: Local Regulators
In other cases, animal cells Communicate using local regulators Only work over a short distance (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
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Long-distance Signaling: Hormones
In long-distance signaling Both plants and animals use hormones 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
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Long-Distance Signaling
Nervous System in Animals Electrical signals through neurons Endocrine System in Animals Uses hormones to transmit messages over long distances Plants also use hormones Some transported through vascular system Others are released into the air
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The Three Stages of Cell Signaling
Signal (Ligand)/Receptor Transduction Response Called Signal transduction pathways Convert signals on a cell’s surface into cellular responses Are similar in microbes and mammals, suggesting an early origin
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Overview of cell signaling
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
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Three Stages of Cell Signaling
EXTRACELLULAR FLUID CYTOPLASM Plasma membrane 1 1 Reception The receptor and signaling molecules fit together (lock and key model, induced fit model, just like enzymes!) Receptor Signaling molecule Signaling molecule binds to the receptor protein
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Three Stages of Cell Signaling
EXTRACELLULAR FLUID CYTOPLASM Plasma membrane 1 1 Reception 2 Transduction Receptor 2nd Messenger! Relay molecules in a signal transduction pathway Signaling molecule The signal is converted into a form that can produce a cellular response
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Three Stages of Cell Signaling
EXTRACELLULAR FLUID CYTOPLASM Plasma membrane 1 Reception 2 Transduction 3 Response Receptor Activation of cellular response Relay molecules in a signal transduction pathway Can be catalysis, activation of a gene, triggering apoptosis, almost anything! Signaling molecule The transduced signal triggers a cellular response
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Signal Transduction Animation
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There are three most common types of membrane receptor proteins.
Ion channel receptors Protein Kinases G-protein coupled receptors
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The G-protein is a common membrane receptor.
1. Reception A signal molecule, a ligand, binds to a receptor protein in a lock and key fashion, causing the receptor to change shape. Most receptor proteins are in the cell membrane but some are inside the cell (intracellular). The G-protein is a common membrane receptor.
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Ion Channel Receptors Very important in the nervous system
Gate closed 1 Ions Signaling molecule (ligand) Very important in the nervous system Signal triggers the opening of an ion channel depolarization Triggered by neurotransmitters Ligand-gated ion channel receptor Plasma membrane 2 Gate open Cellular response 3 Gate closed
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Receptor tyrosine kinases
Protein Kinases Receptor tyrosine kinases 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 ADP Figure 11.7
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G-Protein Coupled Receptors are often involved in diseases such as bacterial infections.
G-Protein Receptors Inactive enzyme Plasma membrane G protein-coupled receptor Activated receptor Signaling molecule Enzyme GDP 1 2 GDP GTP CYTOPLASM G protein (inactive) Activated enzyme i GTP GDP P 3 4 Cellular response
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2. Transduction Transduction: Cascades of molecular interactions relay signals from receptors to target molecules in the cell Multistep pathways Can amplify a signal (Amplifies the signal by activating multiple copies of the next component in the pathway) Provide more opportunities for coordination and regulation At each step in a pathway, the signal is transduced into a different form, commonly a conformational change in a protein.
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Transduction in a G-protein pathway
Fig First messenger Adenylyl cyclase G protein G protein-coupled receptor GTP ATP Second messenger cAMP Transduction in a G-protein pathway Protein kinase A Cellular responses
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Protein Phosphorylation and Dephosphorylation
Many signal pathways Include phosphorylation cascades In this process, a series of protein kinases add a phosphate to the next one in line, activating it Phosphatase enzymes then remove the phosphates
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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
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About 1% of our genes are thought to code for kinases.
The transduction stage of signaling is often a multistep process that amplifies the signal. About 1% of our genes are thought to code for kinases.
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Small Molecules and Ions as Second Messengers
Secondary messengers Are small, nonprotein, water-soluble molecules or ions that act as secondary messengers.
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Calcium ions and Inositol Triphosphate (IP3)
Calcium, when released into the cytosol of a cell acts as a second messenger in many different pathways Calcium is an important second messenger because cells are able to regulate its concentration in the cytosol EXTRACELLULAR FLUID Plasma membrane ATP CYTOSOL Ca2+ pump Endoplasmic reticulum (ER) Nucleus Mitochondrion Key High [Ca2+] Low [Ca2+] Other second messengers such as inositol triphosphate and diacylglycerol can trigger an increase in calcium in the cytosol
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Figure 11.12 3 2 1 4 6 5 EXTRA- CELLULAR FLUID Signal molecule
IP3 quickly diffuses through the cytosol and binds to an IP3– gated calcium channel in the ER membrane, causing it to open. 4 The calcium ions activate the next protein in one or more signaling pathways. 6 Calcium ions flow out of the ER (down their con- centration gradient), raising the Ca2+ level in the cytosol. 5 DAG functions as a second messenger in other pathways. Phospholipase C cleaves a plasma membrane phospholipid called PIP2 into DAG and IP3. A signal molecule binds to a receptor, leading to activation of phospholipase C. EXTRA- CELLULAR FLUID Signal molecule (first messenger) G protein G-protein-linked receptor Various proteins activated Endoplasmic reticulum (ER) Phospholipase C PIP2 IP3 (second messenger) DAG Cellular response GTP Ca2+ (second messenger) IP3-gated calcium channel Figure 11.12
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Cyclic AMP Many G-proteins trigger the formation of cAMP, which then acts as a second messenger in cellular pathways. ATP GTP cAMP Protein kinase A Cellular responses G-protein-linked receptor Adenylyl cyclase G protein First messenger (signal molecule such as epinephrine) Figure 11.10
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Cyclic AMP Cyclic AMP (cAMP) Is made from ATP O –O N O P OH CH2 NH2
H2O HO Adenylyl cyclase Phoshodiesterase Pyrophosphate Cyclic AMP AMP i
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3. Response Many possible outcomes
Growth factor 3. Response Receptor Reception Many possible outcomes This example shows a transcription response Phosphorylation cascade Transduction CYTOPLASM Inactive transcription factor Active transcription factor Response P DNA Gene NUCLEUS mRNA
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Specificity of the signal
Signaling molecule Specificity of the signal The same signal molecule can trigger different responses Many responses can come from one signal! Receptor Relay molecules Response 1 Response 2 Response 3 Cell A. Pathway leads to a single response. Cell B. Pathway branches, leading to two responses.
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The signal can also trigger an activator or inhibitor
The signal can also trigger multiple receptors and different responses Activation or inhibition Response 4 Response 5 Cell C. Cross-talk occurs between two pathways. Cell D. Different receptor leads to a different response.
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Termination of the Signal
Signal response is terminated quickly By the reversal of ligand binding
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Any Questions?? Can You Hear Me Now?
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