Chapter 11: Cell Communication
Chapter 11: Cell Communication Biol 5A
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Date Created: 02/18/16
Chapter 11: Cell Communication General Principles of Cell Signaling Chemical signals can be proteins, amino acids, peptides, nucleotides, steroids, gases Some signals (proteins, amino acids, peptides) are hydrophilic Some signals are hydrophobic (steroid hormones) Signals produced by signal cell and detected by receptor protein on a target cell 1) A wide ranging form of signaling involves a signal produced by the endocrine system called a hormone. Hormones communicate to cells over some distances, produced at one site and act on another. Transported through the bloodstream in animals/sap of plants More localized communication: 2) Paracrine Signaling: One cell secretes local regulators into the extracellular fluid and it acts on nearby target cells Ex. Histamines in inflammatory response, nitric oxide 3) Synaptic/Neuronal signaling: Neuron releases neurotransmitter into a space Neurotransmitters diffuse across the space into neighboring neuron or muscle and activate it, causes a change in the target cell to affect some change or carry the signal in the form of a nerve impulse Messages can be delivered over great distances but occur through private lines at a rapid rate Most direct form of communication: Cell-to-cell communication by plasma membrane o Doesn’t require the release of secreted molecules o Signaling molecules in plasma membrane of one cell bind to receptors embedded in adjacent cells o Also between gap junctions and plasmodesmata General Stages of Cell Signaling Cells must respond selectively to the different signaling chemicals 1) Reception: Ligand molecules bind to a specific cellular protein— receptor—that is located on a target cell Hormones (adrenaline, estradiol, insulin, glucagon), local regulators (PDGF, NO, NGF), and neurotransmitters (acetylcholine) Receptors usually activated by only one signaling molecule and function to take an extracellular signal and convert it into an intracellular signal Most signal receptors are plasma membrane proteins (G protein linked receptor, tyrosine kinase receptor, ion channel receptors) 2) Transduction: Ligand causes a conformation change in receptor protein, causes a series of responses in the cell that convert the original signal into a cellular response 3) Response: Intracellular signaling molecule can eventually cause an enzyme to become activated or cause the expression of a gene G-Protein Coupled Receptor Largest family of receptors but they are not enzymes Signaling molecules can be hydrophilic, water soluble, protein/amino acid derivative When signal molecule binds to G- protein linked receptor, a change of conformation occurs enabling it to interact with an inactive G protein on the intracellular side of the membrane, GDP is displaced by GTP o G proteins are made of 3 protein subunits that are separated from the receptor complexes o G proteins are activated with GTP bound to them. Activated G proteins are able to diffuse along the membrane to find their target proteins Activated G proteins bind to other proteins (enzyme that catalyzes reaction, enzyme that adds P, ion channel) that begin signal transduction pathway. The longer they are bound, the stronger and more prolonged the signal G proteins can be deactivated by the hydrolysis of GTP to GDP. Once GTP is replaced, G protein is inactive and ready to be activated by another signaling molecule Defects in turning on/off the G protein linked receptors are linked to metabolic disorders and diseases Receptor Tyrosine Kinase (enzyme-linked receptor) A kinase is an enzyme that catalyzes the transfer of phosphate groups Tyrosine kinase extends into cytoplasm and catalyzes the transfer of phosphate group from ATP to amino acid tyrosine on substrate protein (attach phosphates to tyrosines) Important in cell division/cell repair, primarily as a growth factor receptor The cytoplasmic side of receptor acts as an enzyme Switched on by the ligand on the extracellular side of the membrane Ligand causes binding of TK receptor subunits to form a dimer The cytoplasmic side of the receptor becomes phosphorylated by ATP Other cellular proteins interact with phosphorylated domain of TK receptor and becomes activated and can initiate numerous transduction systems Termination of signaling processes occurs when protein phosphates (opposite of kinases) remove phosphate groups off of proteins. Tyrosine domain loses phosphates and is no longer active. Activated receptors can be brought into the cell by endocytosis and destroyed with lysosomes. Ligand Gated Ion Channel Serve for rapid transmission across synapses in nervous system Responsible for the conversion of neurotransmitters from outside to an electrical signal inside of the membrane of neurons Binding of ligands causes conformation change in channel that allows specific ions to flow Once channels open, the movement of ions is determined by the electrochemical (concentration, charge) gradients G protein linked receptors and TK receptors function to transmit signals into the cell in form of relay systems formed from intracellular signaling molecules inside of the cell Signaling molecules generate other signals or receive a signal in one part of the cell and move to another part (signaling proteins act as molecular switches) Common mechanism to turn on proteins is the addition of a phosphate group from ATP ATP phosphorylate proteins that are able to begin operation in some pathway, proteins act as switches on the “On” position To stop transduction pathways, the initial signal molecule must be removed from the receptor. Protein phosphatase removes phosphate group from activated protein, turning it off Not all of the components of the transduction pathway are proteins. Some signaling systems rely on small, non-protein, water soluble molecules or ions. These molecules act as secondary messengers, taking information from the primary messenger (the initial signal ligand) and relaying it through the pathway 2nd Messengers: Cyclic AMP Formed from ATP through the removal of 2 phosphate groups using the enzyme adenylyl cyclase Water soluble so it can move easily throughout the cell Messengers binds to the G protein linked receptors and activate the G protein complex Activated G protein activates adenylate cyclase initiating the conversion of ATP to cyclic AMP Cyclic AMP exerts effects within the cell by activating the enzyme it’s bound to Activation of protein kinase A causes other phosphorylation events to occur rapidly (activating CAMP production can cause rapid effects): o Glycogen breakdown in skeletal muscles o Fat breakdown in adipose tissue to fatty acids o Increase in heat rate and force of contraction o Fight or flight responses Other responses are slow, such as the control of gene expression o Hormone—G protein linked receptor—G protein—adenylate cyclase—cAMP —protein kinase A—gene regulatory protein—gene transcription altered 2nd Messengers: Calcium ions and IP3 Muscle contraction, neurotransmitter released by neurons, growth of cytoskeleton Stored in high concentration in ER (actively pumped into ER from cytosol) and released by IP3 when the proper signal is detected Calcium ions enter a cell through gated ion channels o Neurotransmitters bind to channel proteins and the flow of ions occurs. It changes the electrical potential of cells across the membrane Calcium ions can also be added through the IP3 pathway o Ligand binds to the receptor causing a conformation change o Phospholipase C acts on membrane PIP o PIP is hydrolyzed to form 2 molecules: IP3 and DAG o IP3 leaves the plasma membrane and diffuses into the ER where it binds to calcium channels in the ER membrane o Concentration of calcium increased in the cytosol o DAG helps activate another protein kinase which can be used to phosphorylate other proteins Signal molecule—G protein receptor—G protein—activation of phospholipase C— breakdown of PIP IPS—diffuses to ER—opens calcium gated channels—increases calcium concentration in cell—calcium binds to calmodulin (calmodulin has 3 calcium binding sites; it changes its shape and wraps around protein) DAG and IP3 pathways are used for platelet activation, muscle contraction, insulin and amylase secretion, glycogen breakdown Apoptosis Essential for normal development of the nervous system Programmed cell death, when: o DNA damaged/mutated o Caused by a signal o Elimination of cells during development o Does not activate immune system During: o Nuclear DNA fragmented o Cytoskeleton dismantled o Blebbing—fragments of dead cells make up apoptotic bodies taken up by macrophages and used Ced-9: in outer mitochondrial membrane, master regulator of apoptosis, promotes apoptosis when signal is received, activates proteases (breaks proteins/enzymes) and nucleases (breaks DNA) o Main proteases are caspases—Ced-3 is the main caspase Ced-3 and Ced:4 are proteins (activated by death ligand) As long as Ced-9 is active, apoptosis is inhibited and the cell remains alive When the cell receives the death signal, Ced-9 is inactivated, reliving it’s inhibition of Ced-3 and Ced-4. Active Ced-3 triggers cascade of reactions leading to the activation of nucleases and other proteases—eventually leading to cell death Neurosis Swelling destruction of cell after massive injury Signal transduction pathways are a means for a cell to: relay signals from one cell to the cytoplasm of another cell amplify a signal by allowing a few extracellular signals to evoke increased production of secondary messengers diverge the signal by causing pathway to split and be relayed into a number of different targets control activation and inhibition by interfering with any of the steps through mutations, intercellular signals and extracellular signals produce a signal that is recognized by only a specific group of cells with the appropriate receptors rapidly respond to changes (fight or flight, food and smell, photoreceptors in eyes) Figure 1: The Specificity of Cell Signaling
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