Harris Lecture 7 notes (1 per page)

Harris Lecture 7 notes (1 per page) - To understand normal...

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Unformatted text preview: To understand normal cell biology and disease we must understand the molecular machinery that functions inside cells to control their shapes, functions, interactions and numbers. Lectures 1-3: How do cells and tissues organize themselves spatially? 4-6: How do multicellular organisms develop? 7-9: How do cells communicate with each other? 7. Principles of cellular signalling 8. Signalling via small molecules 9. Signalling via protein modifications 10-12: How are cell numbers controlled? 1 1 Cellular signaling Cells must communicate to develop and maintain multicellular organisms Unicellular bacterial-like organisms existed on earth for ~2.5 billion years before complex multicellular organisms arose In part, this delay may have been due to the time needed to evolve complex signaling systems However, unicellular organisms do communicate… 2 2 Examples of unicellular communication Quorum sensing in bacteria: Many bacteria release and respond to chemical signals. This signaling coordinates motility, antibiotic production, spore formation and sexual conjugation in bacterial populations. Mating in budding yeast: Signaling between yeast cells prepares them to mate. signaling leads to elongation of the cell Aggregation of ameboid cells: Signaling between Dictyostelium cells draws them together to form a fruiting body. hunt down bacteria 3 http://www.biocircle.fu-berlin.de/mikrobio2/signaltransduktion en.php?id=mb2&p=9&lang=en 3 How do cells communicate to develop and maintain complex multicellular organisms? ? Mammals, flies and worms use similar communication pathways. core mechanisms Many core pathway components were first discovered in mutants affecting cell communication in Drosophila and C. elegans 4 4 The basics of sending and receiving signals Cells can send out hundreds of different types of signaling molecules (e.g. proteins, small peptides, amino acids, nucleotides, other small molecules and dissolved gases) They are exocytosed, emitted by diffusion, or displayed on the cell surface Cells receive signals in two main ways receptor is inside the cell cannot pass the membrane 5 5 Signaling occurs over short or long distances both the signal and receptor are transmembrane protein. Like Notch - Delta signaling. Contact-dependent signaling The cells have to come into contact with each other. -signals are retained on the cell surface Paracrine signaling -signals are released from the cell but act locally -signal movement is restricted by: 1. internalization by neighbouring cells 2. signal instability or destruction by extracellular enzymes 3. binding to extracellular matrix molecules 6 6 Signaling occurs over short or long distances Synaptic signaling -neurons extend axons to contact distant target cells -the released signaling molecules act locally at target undergo local signaling Endocrine signaling -endocrine cells secrete hormones into the bloodstream for long-range distribution 7 7 Cells use signal transduction pathways to respond to extracellular signals Definitions: Signal transduction > conversion of an extracellular signal into converted into a different molecule the circle is an effector of the triangle the triangle is an effector of the square a intercellular signal Effector anything that is downstream of the pathway A downstream molecule in a signal transduction pathway > 8 8 Cells use signal transduction pathways to respond to extracellular signals Small intracellular signaling molecules are called second messengers -made in large numbers and diffuse through the cytoplasm (e.g. cyclic AMP) or plasma membrane (e.g. diacylglycerol) -bind and alter effector molecules Large and small intracellular signaling molecules are both traducing extracellular signals, but only the small ones are calledLarge intracellular signaling second messenger. molecules are typically proteins -organized into pathways and networks 9 9 Components of signal transduction pathways: Scaffold protein binds to multiple components of the signal traduction pathway together. This increases signaling efficiency and specificity. This enzyme may make small molecules/ this enzyme is a kinase and will phosphorylate downstream proteins. Draws multiple signaling components together to increase efficiency and specificity maybe an enzyme Sends the signal downstream in the same basic form (e.g. protein-protein interactions) coincidence detector attach the signaling component to something restrict the intensity of the signaling 10 10 Components of signal transduction pathways: Converts the signal into a different form (e.g. an enzyme making a small molecule or phosphorylating downstream proteins) Amplifies the signal by performing many conversions. 11 11 Components of signal transduction pathways: Requires inputs from two or more pathways to send the signal on (a coincidence detector) Sends the signal to other pathways (increases the complexity of the response) 12 12 Components of signal transduction pathways: Restricts the location of the signaling (recruitment to a subcellular compartment) Restricts the intensity of the signaling (down-regulation) 13 13 Many components of signaling pathways act like switches Enzymatic versus non-enzymatic conformation Protein-binding versus non-binding conformation Gain or loss of phosphate groups Small molecule binding or unbinding Switching “off” is just as important as switching “on” -if the pathway isn’t turned off, it won’t be able to send another signal 14 14 Addition and removal of phosphate groups on proteins This signaling is very common! ~30% of human proteins are phosphorylated activate protein kinase The human genome encodes ~520 protein kinases and ~150 protein phosphatases phosphate added There are serine/threonine kinases and tyrosine kinases Serine, threonine and tyrosine have an OH group that the kinase can add the P onto. 1. If the green molecule is an enzyme, it's active. 2. P may cause a conformational change in the protein and bind to another protein 15 15 GTP or GDP binding GTP binding proteins -Large trimeric and small monomeric types loss of GDP gain of GTP -Have low GTPase activity 16 16 GTP or GDP binding GTPase activating proteins (GAPs) increase the GTP hydrolysis Guanine nucleotide exchange factors (GEFs) promote the exchange of GDP for GTP 17 17 Many components of signaling pathways interact like Lego Proteins often contain one or more interaction domains These bind to structural motifs in other molecules (short peptide sequences, other domains, covalent modifications) During evolution, domains can be added or removed from proteins to alter interactions and re-wire signaling pathways Some examples: SH2 and PTB domains bind phosphotyrosine containing sequences one signal polypeptide chain folded into 3 domains PH domains bind phosphoinositides SH3 domains bind prolinerich sequences 18 18 How are complex signaling events orchestrated? How is signaling specificity achieved? How are signaling pathways coordinated? How are signaling pathways organized in space and time? 19 19 Targeting specific cells Synaptic signaling specificity: Neurons make connections with specific target cells (the same signaling molecules can be used at all connections) Endocrine signaling specificity: Different molecules are released and target cells express specific receptors to respond to specific molecules neurons send out axons to make specific connections with target cells. they can use the same molecules because neuronal connections gave specificity already. target cells have different receoptors 20 20 Different cells can also have different responses to the same molecule by changing the signal receptor or downstream components of the pathway specificity can be determined by receptor, not molecule. 21 21 Within a cell, a signal transduction molecule will often function in many different pathways What prevents an upstream signal from activating all of the pathways? PNAS 104:12890 22 22 Within a cell, a signal transduction molecule will often function in many different pathways What prevents an upstream signal from activating all of the pathways? The formation of local complexes helps insulate pathways from each other 23 23 Within a cell, a signal transduction molecule will often function in many different pathways What prevents an upstream signal from activating all of the pathways? The formation of local complexes helps insulate pathways from each other 24 24 Within a cell, a signal transduction molecule will often function in many different pathways What prevents an upstream signal from activating all of the pathways? The formation of local complexes helps insulate pathways from each other 1 can only phosphorylate 2 because of the scaffold 25 25 How are complex signaling events orchestrated? How is signaling specificity achieved? How are signaling pathways coordinated? How are signaling pathways organized in space and time? 26 26 Cellular outcomes often depend on multiple signaling inputs get the message from many different ways then differentiate 27 27 Coincidence detectors only activate downstream signals when two upstream signals are both detected This ensures two conditions are met before the cell responds 28 28 How are complex signaling events orchestrated? How is signaling specificity achieved? How are signaling pathways coordinated? How are signaling pathways organized in space and time? 29 29 How are complex signaling events orchestrated? How is signaling specificity achieved? How are signaling pathways coordinated? How are signaling pathways organized in space and time? In addition to forming local complexes, signaling pathways can be organized at subcellular compartments such as… …primary cilia… Primary cilia is microtubulebased protrusion. Each cell has 1 cilia. Receptors for downstream components are all housed in this primary cilia. …or synapses 30 biochemistry.ucsf.edu/labs/reiter/index-home.shtml cumc.columbia.edu/publications/press_releases/STVimaging.html 30 Signaling also occurs over different time frames Synaptic signaling is very fast (electrical impulses travel at 100 m/s) Endocrine signaling is relatively slow (limited by blood flow) 31 31 if the protein has been made already and just need to be phosphorylated, or change conformational Signaling also occurs over different time frames The response speed to a signal can also be slow or fast depending on the cellular machinery involved Also, the speed with which the cell can turn off signaling depends on the stability molecules involved 32 if the molecules turns over quickly(unstable), it will be easy to turn the signal off. 32 If the protein has not been made, signal goes into nucleus promotes transcription and translation. Cellular outcomes also depend on feedback mechanisms A and B are self supporting. 33 33 Effects of positive feedback There is activity only when a signal is present. A, B are activating each other. Self-sustaining. Bi stable system(signal on/off state) 34 34 Effects of positive feedback The feedback will enhance the response If the feedback is strong enough, it can be self-sustaining and maintain the system at high activation even with loss of the original signal a ‘bistable’ system that can stably exist in ‘off’ or ‘on’ states 35 35 Cellular outcomes also depend on feedback mechanisms Biphasic switches promote differentiation Positive feedback: the cell initially receives a signal to differentiate. After signal is lost, positive feedback loop is sustaining the signal, so that the cell remembers to differentiate. 36 36 Effects of negative feedback There is an initial signal, but the signal will die down. If feedback occurs quickly, the signaling is suppressed The system adapts to the activation and needs boosted activation to signal more needs a higher amount of signalling This allows cells to respond to changes in upstream signals rather than their absolute amounts (Thus cells can respond to a wide range of signal strengths) 37 37 Effects of negative feedback If feedback occurs slowly, then the system can oscillate (e.g. during circadian rhythms) 38 38 To understand normal cell biology and disease we must understand the molecular machinery that functions inside cells to control their shapes, functions, interactions and numbers. Lectures 1-3: How do cells and tissues organize themselves spatially? 4-6: How do multicellular organisms develop? 7-9: How do cells communicate with each other? 7. Principles of cellular signalling 8. Signalling via small molecules 9. Signalling via protein modifications 10-12: How are cell numbers controlled? 39 39 Lecture 7 reading 879-883 884-886 (not figure 15-7) 893-899 901-902 40 40 ...
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This note was uploaded on 04/03/2012 for the course BIOLOGY BIO230 taught by Professor Harris during the Fall '11 term at University of Toronto.

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