10/15/2011. Chapter 11 Cell Communication. Outline. Overview: Cellular Messaging. Evolution. Evolution of Signaling

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1 Chapter 11 Cell Communication Outline I. Cell Signaling II. Forms of cell signaling III. Quick review of cell membrane IV. Cell Surface s I. G- Coupled s II. osine Kinase s III. Ligand-Gated Ion Channels V. Intracellular s I. Down regulation II. Apoptosis Overview: Cellular Messaging Figure 11.1 Cell-to-cell communication is essential for both multicellular and unicellular organisms Biologists have discovered some universal mechanisms of cellular regulation Cells most often communicate with each other via chemical signals For example, the fight-or-flight response is triggered by a signaling molecule called epinephrine Evolution Fossil record indicate that one celled bacteria were present on earth 3.5 billion years ago But it took another 2.5 billions years for multicellular organisms to appear in the fossil record What took so long for multicellular life to evolve? Evolution of Signaling athway similarities suggest that ancestral signaling molecules evolved in prokaryotes and were modified later in eukaryotes The concentration of signaling molecules allows bacteria to sense local population density 1

2 Local and Long-Distance Signaling Figure 11.4 lasma membranes Cells in a multicellular organism communicate by chemical messengers Animal and plant cells have cell junctions that directly connect the cytoplasm of adjacent cells Gap junctions between animal cells (a) Cell junctions lasmodesmata between plant cells In local signaling, animal cells may communicate by direct contact, or cell-cell recognition (b) Cell-cell recognition Local and Long-Distance Signaling Forms of Signaling In many other cases, animal cells communicate using local regulators, messenger molecules that travel only short distances In long-distance signaling, plants and animals use chemicals called hormones The ability of a cell to respond to a signal depends on whether or not it has a receptor specific to that signal 1. Gap Junctions 2. Autocrine A cell secretes a molecule that binds back onto its own receptor 3. aracrine Local mediators 4. Synaptic Nerve cell signal transmission Neurotransmitters, released into synaptic cleft 5. Endocrine Hormones, secreted into the bloodstream Figure 11.5a Figure 11.5b Long-distance signaling Local signaling Target cell Electrical signal along nerve cell triggers release of neurotransmitter. Endocrine cell Blood vessel Secreting cell Secretory vesicle Neurotransmitter diffuses across synapse. Hormone travels in bloodstream. Local regulator diffuses through extracellular fluid. Target cell is stimulated. Target cell specifically binds hormone. (a) aracrine signaling (b) Synaptic signaling (c) Endocrine (hormonal) signaling 2

3 The Three Stages of Cell Signaling Earl W. Sutherland discovered how the hormone epinephrine acts on cells Sutherland suggested that cells receiving signals went through three processes Reception Transduction Response Animation: Overview of Cell Signaling Right-click slide / select lay Figure Figure EXTRACELLULAR lasma membrane CYTOLASM EXTRACELLULAR lasma membrane CYTOLASM 1 Reception 1 Reception 2 Transduction Relay molecules in a signal transduction pathway Signaling molecule Signaling molecule Figure Transduction: Cascades of molecular interactions relay signals from receptors to target molecules in the cell EXTRACELLULAR Signaling molecule lasma membrane CYTOLASM 1 Reception 2 Transduction 3 Response Relay molecules in a signal transduction pathway Activation of cellular response Signal transduction usually involves multiple steps Multistep pathways can amplify a signal: A few molecules can produce a large cellular response Multistep pathways provide more opportunities for coordination and regulation of the cellular response 3

4 Fig. 9.8.b Signal Transduction athways The molecules that relay a signal from receptor to response are mostly s Like falling dominoes, the receptor activates another, which activates another, and so on, until the producing the response is activated At each step, the signal is transduced into a different form, usually a shape change in a rotein hosphorylation rotein Dephosphorylation In many pathways, the signal is transmitted by a cascade of phosphorylations rotein kinases transfer phosphates from AT to, a process called phosphorylation rotein phosphatases remove the phosphates from s, a process called dephosphorylation This phosphorylation and dephosphorylation system acts as a molecular switch, turning activities on and off or up or down, as required Figure Figure 11.10a Signaling molecule Activated relay molecule Activated relay molecule kinase 1 Active kinase 1 kinase 2 i AT AD kinase 3 i AT Active kinase 2 AD i AT Active kinase 3 AD Active Cellular response kinase 1 Active kinase 1 kinase 2 i AT AD kinase 3 i AT Active kinase 2 AD i AT Active kinase 3 AD Active 4

5 Fig. 9.3 Which type of ligand would bind to a receptor in the plasma membrane 1. Water soluble 2. Lipid soluble 50% 50% 1 2 Copyright 2009 earson Education, Inc. The following can freely pass through a membrane: Fig Ions 2. Hydrophobic molecules 3. hydrophillic molecules 4. Ions and hydrophilic molecules 25% 25% 25% 25% Copyright 2009 earson Education, Inc. Reception: A signaling molecule binds to a receptor, causing it to change shape The binding between a signal molecule (ligand) and receptor is highly specific A shape change in a receptor is often the initial transduction of the signal Most signal receptors are plasma membrane s s in the lasma Membrane Most water-soluble signal molecules bind to specific sites on receptor s that span the plasma membrane = cell surface receptors There are three main types of membrane receptors G -coupled receptors tyrosine kinases Ion channel receptors 5

6 s in the lasma Membrane - GCRs Figure 11.7a G -coupled receptors (GCRs) are the largest family of cell-surface receptors Signaling molecule binding site A GCR is a plasma membrane receptor that works with the help of a G Segment that interacts with G s G -coupled receptor G-Coupled G roteins Trimeric GT-binding = G roteins G roteins are either in the active or inactive state Active state has GT bound When the ligand binds the G- Coupled, this causes a change in shape, that activates the G rotein This starts a chain of events that involve intracellular mediators = secondary messengers Figure 11.7b Trimeric G roteins G -coupled receptor lasma membrane Activated receptor Signaling molecule enzyme G roteins are composed of three s: α and β and γ subunits CYTOLASM 1 GD G (inactive) Enzyme Activated enzyme 2 GD GT GD GT When the G rotein is activated (GT is bound) the α subunit (with GT) goes one way and the β and γ go together another way. GT 3 Cellular response 4 GD i β and γ are active when they are not attached to the α subunit the α subunit has GTase activity 6

7 GTase activity in G s Fig The G is active only when it has GT bound to it. The G has GTase activity, which means it will automatically hydrolyze a phosphate and have GD bound. This way it inactivates itself automatically This GTase activity is enhanced by GTase Activating roteins (GAs) GCR video Small Molecules and Ions as Second Messengers The extracellular signal molecule (ligand) that binds to the receptor is a pathway s first messenger Second messengers are small, non, water-soluble molecules or ions that spread throughout a cell by diffusion Second messengers participate in pathways initiated by GCRs and RTKs Cyclic AM and calcium ions are common second messengers Cyclic AM Figure 11.11a Cyclic AM (cam) is one of the most widely used second messengers Adenylyl cyclase, an enzyme in the plasma membrane, converts AT to cam in response to an extracellular signal Adenylyl cyclase yrophosphate i AT cam 7

8 Figure 11.11b Figure First messenger (signaling molecule such as epinephrine) G Adenylyl cyclase hosphodiesterase G -coupled receptor GT H 2O H 2O AT cam Second messenger cam AM rotein kinase A Cellular responses cam athway 1. A ligand binds to the G- Coupled (GCR). 2. The binding of the ligand (hormone) activates the GCR. 3. The active GCR is able to bind the G- 4. The G- ejects a GD and accepts a GT molecule. The G- is now active 5. The α subunit of the G- with the GT disassociates from the β and γ subunits 6. The α subunit of the G- with the GT goes to adenylyl cyclase and activates it 7. The active adenylyl cyclase transforms AT into cam 8. cam activates a kinase A (KA) 9. cam is inactivated by phosphodiesterases 10. The KA phosphorylates s 11. The phosphorylated s are now active and can change the cell activity 12. The g- with GT bound will hydrolyze the phosphate from GT, now has GD bound and is inactive. It will reform with the β and γ subunits and this inactivates them Fig Fig

9 Figure Reception Binding of epinephrine to G -coupled receptor (1 molecule) Gene transcription by cam Transduction G Active G (10 2 molecules) The cam pathway can also activate the transcription of specific genes adenylyl cyclase Active adenylyl cyclase (10 2 ) AT Cyclic AM (10 4 ) kinase A Active kinase A (10 4 ) phosphorylase kinase Active phosphorylase kinase (10 5 ) The kinase A (KA) activated by cam can turn on transcription of DNA using a CRE-binding (cam response element) glycogen phosphorylase Active glycogen phosphorylase (10 6 ) Response Glycogen Glucose 1-phosphate (10 8 molecules) Examples of Hormone induced responses mediated by cam Target Tissue Hormone Major Response Adrenal Cortex ACTH Cortisol Secretion Ovary LH rogesterone secretion Muscle Adrenaline Glycogen breakdown Bone arathyroid Bone reabsorption hormone (TH) Heart Adrenaline Increase heart rate Liver Glucagon Glycogen breakdown Kidney Vasopressin Water reabsorption Fat Adrenaline, ACTH, glucagon Triglyceride breakdown Animation: Signal Transduction athways Right-click slide / select lay Calcium Ions and Inositol Triphosphate (I 3 ) Figure EXTRACELLULAR lasma membrane Calcium ions (Ca 2+ ) act as a second messenger in many pathways Calcium is an important second messenger because cells can regulate its concentration CYTOSOL AT Ca 2 pump Mitochondrion Nucleus AT Ca 2 pump Ca 2 pump Endoplasmic reticulum (ER) Key High [Ca 2 ] Low [Ca 2 ] 9

10 Calcium Ions I 3 and DAG A signal relayed by a signal transduction pathway may trigger an increase in calcium in the cytosol athways leading to the release of calcium involve inositol triphosphate (I 3 ) and diacylglycerol (DAG) as additional second messengers I3/DAG athway Remember that the plasma membrane is a double layer of phospholipids. It is a mixture of different phospholipids. One of the phospholipids in the membrane is I 2 in the membrane. Figure EXTRA- CELLULAR Signaling molecule (first messenger) G DAG G -coupled receptor GT hospholipase C I 2 I 3 (second messenger) I 3-gated calcium channel Endoplasmic reticulum (ER) Ca 2 CYTOSOL Figure Figure EXTRA- CELLULAR Signaling molecule (first messenger) G EXTRA- CELLULAR Signaling molecule (first messenger) G DAG DAG G -coupled receptor GT hospholipase C I 2 I 3 (second messenger) G -coupled receptor GT hospholipase C I 2 I 3 (second messenger) I 3-gated calcium channel I 3-gated calcium channel Endoplasmic reticulum (ER) CYTOSOL Ca 2 Ca 2 (second messenger) Endoplasmic reticulum (ER) CYTOSOL Ca 2 Ca 2 (second messenger) Various s activated Cellular responses 10

11 Fig Copyright The McGraw-Hill Companies, Inc. ermission required for reproduction or display. Ca 2+ Ca 2+ Ca 2+ Calmodulin Calmodulin Active a. b. I 3 and DAG athway 1. A ligand binds to the G- Coupled (GCR). 2. The binding of the ligand (hormone) activates the GCR. 3. The active GCR is able to bind the G- 4. The G- ejects a GD and accepts a GT molecule. The G- is now active 5. The α subunit of the G- with the GT disassociates from the β and γ subunits 6. The α subunit of the G- with the GT bound goes to phospholipase C (LC) and activates it 7. The active LC splits the phospholipid I 2 into two parts: I3 and DAG. 8. I 3 causes the endoplasmic reticulum to release Ca Ca ++ binds to s like calmodulin 10. Calmodulin with Ca ++ bound activates other s 11. DAG activates kinase C (KC), Ca ++ is needed for the activation 12. Active KC phosphorylates s 13. hosphorylated s are active and alter cell activity 14. The g- with GT bound will hydrolyze the phosphate from GT, now has GD bound and is inactive. It will reform with the β and γ subunits and this inactivates them 15. The cell stores Ca ++ in organelles and pumps it out of the cell Cellular responses mediated by I 3 Target Tissue Signaling Major Response Molecule Liver Vasopressin Glycogen Breakdown ancreas Acetylcholine Amylase secretion Smooth Muscle Acetylcholine Contraction Mast cells Antigen Histamine Secretion Blood latelets Thrombin Aggregation 11

12 Enzyme-Linked Cell Surface s tyrosine kinases (RTK) hosphorylates tyrosine amino acids on signaling s Serine/threonine kinases hosphorylates serine or threonine amino acids on signaling s tyrosine kinases (RTKs) tyrosine kinases (RTKs) are membrane receptors that attach phosphates to tyrosines A receptor tyrosine kinase can trigger multiple signal transduction pathways at once Abnormal functioning of RTKs is associated with many types of cancers osine Kinase These receptors just cross the plasma membrane once single transmembrane s RTKs are dimers it takes two RTKs to function Binding of the ligand causes two RTKs to come together and link to form a dimer = dimerize osine Kinase When the two receptors link together, they now have enzyme activity They phosphorylate tyrosine residues on each other = autophosphorylation This requires AT osine Kinase Ligands Figure 11.7c Signaling molecule (ligand) helix in the membrane Ligand-binding site Signaling molecule Examples of ligands for receptor tyrosine kinases: Insulin Growth hormone Erythropoietin 1 osines CYTOLASM tyrosine kinase s (inactive monomers) 2 Activated relay s Dimer 6 AT 6 AD Activated tyrosine Fully activated kinase regions receptor tyrosine (unphosphorylated kinase dimer) (phosphorylated 3 dimer) 4 relay s Cellular response 1 Cellular response 2 12

13 Fig. 9.7 MA Cascades Mitogen-activated kinases (MA) Mitogens are important in normal cell division MA kinases are a series of kinases that phosphorylate other kinases Ending in a cellular response including gene transcription These cascades amplify the response MA Cascades Fig. 9.8.a The process starts with a growth factor binding to a RTK RTK dimerizes and autophorylates The activated RTK bind activator s The activator s (GRB2 and SOS) activate a GT-binding s (G ) called Ras When Ras is activated, it releases GD and accepts GT Ras activates MKKK, which activates MKK Fig. 9.8.b 13

14 Fig Figure Growth factor Reception hosphorylation cascade Transduction CYTOLASM transcription factor Active transcription factor Response DNA Gene NUCLEUS mrna Summary of RTK/Ras/MA athway 1. Two ligands binds to two RTKs (one ligand/receptor) 2. The two RTK dimerize and phosphorylate each other, activating each other 3. The active RTKs activate a that stimulates Ras to eject GD and accept GT 4. The now active Ras activates the MA kinase cascade until the MA kinase is activated 5. The MA kinase activates s which leads to cellular activity. 6. Ras will automatically inactivate by dephosphorylating the GT to GD 14

15 Ligand-gated ion channel Figure 11.7d A ligand-gated ion channel receptor acts as a gate when the receptor changes shape When a signal molecule binds as a ligand to the receptor, the gate allows specific ions, such as Na + or Ca 2+, through a channel in the receptor Signaling molecule (ligand) Gate closed Ions lasma Ligand-gated membrane ion channel receptor Gate open Cellular response Gate closed Intracellular s Intracellular s Intracellular receptor s are found in the cytosol or nucleus of target cells Small or hydrophobic chemical messengers can readily cross the membrane and activate receptors Examples of hydrophobic messengers are the steroid and thyroid hormones of animals An activated hormone-receptor complex can act as a transcription factor, turning on specific genes Lipid soluble and small signaling molecules bind to receptors located inside the cell. Intracellular receptors are located in the cytosol or the nucleus The intracellular receptors in the cytosol will bind with the ligand, the binding will cause the receptor/ligand complex to travel to the nucleus. Examples of Intracellular s Steroid hormones Estrogen, testosterone, cortisol Effects gene expression Nitric Oxide (gas) Binds to receptor that is an enzyme = guanylyl cyclase Steroid Hormone Intracellular s These hormones cross the plasma membrane and bind to an intracellular receptor in the cytosol The binding of the ligand to the receptor causes the ligand/receptor to go to the nucleus The ligand/receptor regulates gene expression 15

16 Intracellular s Intracellular s Intracellular receptors have three regions: Ligand binding domain DNA binding domain Transcription activating domain rior to the ligand binding, the receptor may have s that block the DNA binding domain. The binding of the ligand signals the receptor to go the nucleus and allow the receptor to bind to the DNA Fig. 9.5 Figure Hormone (testosterone) EXTRACELLULAR lasma membrane DNA NUCLEUS CYTOLASM Figure Hormone (testosterone) EXTRACELLULAR Figure Hormone (testosterone) EXTRACELLULAR lasma membrane Hormonereceptor complex lasma membrane Hormonereceptor complex DNA DNA NUCLEUS NUCLEUS CYTOLASM CYTOLASM 16

17 Figure Hormone (testosterone) EXTRACELLULAR Figure Hormone (testosterone) EXTRACELLULAR lasma membrane Hormonereceptor complex lasma membrane Hormonereceptor complex DNA DNA mrna mrna NUCLEUS NUCLEUS New CYTOLASM CYTOLASM Nuclear and Cytoplasmic Responses Figure RESULTS Ultimately, a signal transduction pathway leads to regulation of one or more cellular activities The response may occur in the cytoplasm or in the nucleus Many signaling pathways regulate the synthesis of enzymes or other s, usually by turning genes on or off in the nucleus The final activated molecule in the signaling pathway may function as a transcription factor CONCLUSION 1 Mating factor activates receptor. GD Wild type (with shmoos) Fus3 formin Mating factor G -coupled receptor Shmoo projection forming Formin Fus3 Actin GT subunit 2 G binds GT and becomes activated. hosphorylation Formin Formin cascade 4 Fus3 phosphorylates Fus3 Fus3 formin, Microfilament activating it. 5 Formin initiates growth of 3 hosphorylation cascade microfilaments that form activates Fus3, which moves the shmoo projections. to plasma membrane. Fine-Tuning of the Response Signal Amplification There are four aspects of fine-tuning to consider Amplification of the signal (and thus the response) Specificity of the response Overall efficiency of response, enhanced by scaffolding s Termination of the signal Enzyme cascades amplify the cell s response At each step, the number of activated products is much greater than in the preceding step 17

18 The Specificity of Cell Signaling and Coordination of the Response Different kinds of cells have different collections of s These different s allow cells to detect and respond to different signals Even the same signal can have different effects in cells with different s and pathways athway branching and cross-talk further help the cell coordinate incoming signals Figure Signaling molecule Relay Activation molecules or inhibition Response 1 Response 2 Response 3 Response 4 Response 5 Cell A. athway leads Cell B. athway branches, Cell C. Cross-talk occurs Cell D. Different receptor to a single response. leading to two responses. between two pathways. leads to a different response. Signaling Efficiency: Scaffolding roteins and Signaling Complexes Figure Scaffolding s are large relay s to which other relay s are attached Scaffolding s can increase the signal transduction efficiency by grouping together different s involved in the same pathway In some cases, scaffolding s may also help activate some of the relay s Signaling molecule Scaffolding lasma membrane Three different kinases Termination of the Signal Internalization Inactivation mechanisms are an essential aspect of cell signaling If ligand concentration falls, fewer receptors will be bound Unbound receptors revert to an inactive state One way this pathway is inactivated is through Ras GTase activity Another way this pathway is down-regulated is internalization of the receptors We will come back to this at the end of the lecture 18

19 Controlling Cell Signaling To control cell signaling: 1. The G-s have GTase activity 2. Ligand levels fall 3. Ca ++ is sequestered 4. There are phosphatases that inactivate the s that were activated by phosphorylation 5. hosphodiesterases 6. s are internalized Apoptosis integrates multiple cell-signaling pathways Figure Apoptosis is programmed or controlled cell suicide Components of the cell are chopped up and packaged into vesicles that are digested by scavenger cells 2 m Apoptosis prevents enzymes from leaking out of a dying cell and damaging neighboring cells 19

20 Apoptotic athways and the Signals That Trigger Them Mitochondria release cytochrome C Caspases are the main proteases (enzymes that cut up s) that carry out apoptosis Apoptosis can be triggered by An extracellular death-signaling ligand DNA damage in the nucleus rotein misfolding in the endoplasmic reticulum Apoptotic athways Apoptosis evolved early in animal evolution and is essential for the development and maintenance of all animals Apoptosis may be involved in some diseases (for example, arkinson s and Alzheimer s); interference with apoptosis may contribute to some cancers Figure Apoptosis video Interdigital tissue Cells undergoing apoptosis 1 mm Space between digits Florescent labeled Death ligand Important Concepts Know the vocabulary covered in this lecture Know examples of signaling molecules Know the forms of cell signaling Know the main classifications of signaling receptors, what are the main differences and what type of ligands generally bind these receptors Understand how intracellular receptors work, what are the steps from the binding of a ligand to the cellular activity and what type of activity is regulated by intracellular receptors. Important Concepts Know the three domains of intracellular receptors Know the classes of Cell Surface roteins Understand how kinases work, what do they do? Know the properties of RTKs. Be able to describe the steps of how the receptors regulate cellular activity What is the benefit of having kinase cascades? 20

21 Important Concepts Know the examples of ligands for the receptors Be able to describe how cellular activity is regulated by the cam pathway, starting with the binding of a ligand to the GCR, including how the g- is inactivated Know examples of a cellular response mediated by cam Understand the apoptotic pathways and how they are triggered Important Concepts Be able to describe the steps of how cellular activity is effected by the I3 and DAG pathway, starting with the binding of a ligand to the GCR, including how the g- is inactivated Know examples of cellular responses mediated by I 3 How are cell signaling pathways controlled What are differences and similarities between G- couple receptors, enzyme linked receptors (like RTK) and intracellular receptors 21