Lecture 18: Cell Communication Flashcards
1
Q
Extracellular signaling molecules bind to specific receptors in target cells to initiate a chain of events referred to as
A
Signal transduction
2
Q
External signals induce what 2 major types of responses
A
- Change in activity or function of enzymes or proteins in the cell (Fast response)
- Change in amounts of proteins by change in expression of genes (slow response)
3
Q
____ is released from fat and signals hypothalamus that you are full
A
Leptin
4
Q
Types of cell signaling
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- Endocrine signaling
- Paracrine signaling
- Autocrine signaling
- Direct Signaling
5
Q
Endocrine Signaling
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- Long distance signaling
- Signal—> bloodstream —-> distant target cells
- Freely diffusible signals
- Long lasting (long half-life in minutes)
- takes time to go through the circulatory system to find a target cell
6
Q
Paracrine signaling
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- Acts locally
- Affects cells nearby (not as freely diffusible)
- Short lived signal
- e.g. neurotransmitters
7
Q
Autocrine Signaling
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- Cells respond to signals that they themselves release or release to cells of the same type
- Cell secretes signal that feeds back and binds to a receptor on its own surface
- e.g. growth factors in cancer cells
8
Q
Direct cell signaling
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- also known as juxtacrine signaling
- e.g. immune cells
- Ag-presenting cells to T cells
9
Q
Does each cell interpret the combination of all types of signaling to determine actions
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Yes
10
Q
Examples of Acetylcholine having different effects on different types of cells (ex. of same ligand-different responses)
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- relaxs heart muscle cells
- contracts skeletal muscle cells
- causes the secretion of saliva by salivary gland cells
11
Q
How does signal Transduction work
A
12
Q
what are the componets of the cell signaling cascade
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- Signals (ligands)
- Typically secreted by exocytosis (e.g. signal peptide)
- Signals stay near or far
- Receptors
- Bind specifically to signal molecules with high affinity (signals are produced in low levels)
- Effectors
- Targets of receptors inside cells: alter activity of many different proteins and generate 2nd messenger (small diffusible molecules like cAMP and Ca2+)
13
Q
Cell Signaling: ligands
A
- can be proteins, small peptides, amino acid derivatives, hydrophobic molecules (steroid hormones like estrogen), and even gases (NO)
- Main categories:
- Small lipophilic molecules: steroid hormones
- water soluble molecules (hydrophilic)
- e.g. growth factors
14
Q
Examples of Lypophilic (“lipid-loving”) signaling molecules (ligands)
A
- Steroid hormones: progesterone, estradiol, testosterone, cortisol, aldosterone, vitamin D
- Thyroid hormone: Thyroxine
- Retinoids: retinol, retinoic acid
15
Q
Receptor location and type for lypophilic (“lipid-loving”) ligands
A
- Found in the cytoplasm and nucleus
- Family of DNA-binding transcription factors
16
Q
Examples of hydrophilic (“water-loving”) signaling molecules (ligands)
A
- Amino acid derived:
- histamine, serotonin, melatonin, dopamine, norepinephrine, epinephrine
- From lipid metabolism:
- acetylcholine
- Polypeptides:
- insulin, glucagon, cytokines, thyroid-stimualting hormone
17
Q
Receptor location and type for hydrophilic (“water-loving”) ligands
A
- Found on the surface of plasma membranes
- includes transmembrane proteins such as G protein-coupled receptors and receptor tyrosine kinases
18
Q
What are the two general types of receptors
A
- Intracellular receptors
- steroid receptor can have receptor in cytosol (e.g. estrogen) - atlers gene expression in nucleus
- Cell surface receptors
- external domain binds ligand
- transmembrane domain anchors receptor, cytoplasmic domain initiates signal by change in conformation
19
Q
most signaling molecules are
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hydrophilic and require cell-surface receptors
20
Q
A
21
Q
What are the three main types of cell signaling receptors in the Plasma membrane
A
- Ion-channel-coupled receptors
- G-Protein Coupled receptors
- Enzyme-coupled receptors
22
Q
G-protein coupled receptors use ___ pass transmembrane proteins
A
7
23
Q
whaat type of signaling recepotrs are common in nervous tissue
A
Gated ion
24
Q
______ receptors include receptor tyrosine kinases (RTKs)
A
Enzyme-coupled receptor class
25
Transmembrane receptors
* Receptor mediated signaling
* most ligands or hormones are hydrophilic or large and can't get into a cell
* They need some way to transduce a binding event on the cell surface to send signal inside the cell
* One major class of surface receptors that mediate these signals are G-protein coupled receptors (also called 7 transmembrane receptors)
* There are \> 1,000 G-protein coupled receptors (also called 7 transmembrane receptors)
* Affect olfaction, sight, taste
* GPCRs are targets of many drugs (60% of all drugs on the market or being tested target G-protein coupled receptors)
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G-Protein coupled receptors
* Receptor composed fo 3 parts
* extracellular domain
* binds ligand
* Transmembrane domain
* anchors receptor
* Cytoplasmic domain
* associates with G-proteins
* Regulate target enzyme
* No intrinsic catalytic activity
* Heterotrimeric G-proteins are guanine nucleotide-binding proteins that consist of three subunits designated alpha, beta, and gamma
* 60% of all drugs on market and being tested target GPCRs
* Activity
* GPCR---\> trimeric G protein ---\> Effector Enzyme ----\> 2nd messenger -----\> targets of 2nd messenger ----\> biological process
* GPCR do not transfer phosphates (not active kinases)
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* In an unstimulated state, the alpha subunit of GPCR has GDP
* when a GPCR is activated, it acts like a guanine nucleotide expchange factor (GEF) and induces the alpha subunit to release its bound GDP, allowing GTP to bind in its place.
* GTP biding then causes an activating conformational change in the Galpha subunit, releasing the G protein from the receptor and triggering dissociation of the GTP-bound Galpha subunit from the Gbetagamma pair. Both of which then interact with various targets, such as enzymes and ion channels in the PM, which relay the signal onward.
* GDP is bound to the alpha subunit and the G protein is inactive
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The alpha subunit in GPCR is a GTPase and becomes inactive when
it hydrolyzes its bound GTP to GDP
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Steps of G-protein relaying signals
* Ligand binds to receptor
* Conformational change occurs in receptor
* Receptor binds to G protein
* Receptor then acts as a GEF (guanine exchange factor)
* Confirmation of Galpha protein is changed such that it kicks out GDP and GTP binds to it
* Galpha is now active and dissociates from beta and Gamma and can now bind to effector molecule and activate efector molecule
* effector molecule in this case is adenylyl cyclase, which catalyzes formation of cAMP
* Eventually hydrolysis of GTP bound to Galpha occurs and changes to GDP (occurs after a certain amount of time)
* Galpha returns to inactive step to be recycled through process again
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cAMP targets
* cAMP activates cAMP-dependent protein kinase (PKA)
* 4 subunits
* inactive PKA
* 2 catalytic subunits
* 2 regulatory subunits
* binding of 2 cAMP molecules to regulatory subunits of tetramer results in release of active C subunits
* active PKA can now phosphorylate other proteins
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Consequences of protein phosphorylation by PKA
* PKA can regulate proteins by addition of phosphate groups: addition of 2 negative charges can change conformation of protein
* Phosphate group can form part of structure that other proteins recognize
* Activation or inactivation of enzymatic target proteins
* Alteration of intracellular localization of target proteins
* Alterations in abundance of target proteins
35
36
What is the molecular mechanism by which cholera toxin acts
* Toxin targets a G-protein
* modifies G protein by keepin the Galpha in the GTP active form indefinitely (Covalently modifies)
* this leads to 100 fold increase in cAMP
* PKA phosphorylates the CFTR Cl- channel
* this leads to secretion of water
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The ability to turn off or reject the signal
Desensitization (this is a very important characteristic think about cancer)
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what are potentiate and attenuate
* Potentiate= turn up
* Attenuate= turn down
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Ways to attenuate or desensitize signal
* Hormone levels drop
* decreased adenylyl cyclase activity
* thus decrease cAMP
* thus decrease in PKA activity
* Remove the signaling molecule:
* phosphodiesterases will remove cAMP/cGMP
* Receptor sequestration
* endosome
* Receptor destruction
* endosomes + lysosomes (proteases)
* GRKs (G protein receptor kinases)
* phosphorylate the receptor such that another protein called arrestin will bind to the 3rd intracellular loop and prevents Ga from interacting with the third loop
* Result is that Galpha-GDP does not get converted to Galpha-GTP
40
What are GRKs
* GRKS (G protein receptor kinases)
* phosphorylate the receptor such that another protein called **arrestin** will bind to the 3rd intracellular loop and prevents Ga from interacting with the third loop
* result is that Galpha-GDP does not get converted to Galpha-GTP
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Gi/oalpha G-protens function
inhibit Adenylate cyclase and thus cAMP is not produced and PKA is not activated
42
Gqalpha G-proteins function
* Activates PLC instead of Adenylate cyclase
* PLC cleaves PIP2 into IP3 (inositol 1,4,5-triphosphate (diffusible)) and DAG (1,2-diacylglycerol)(these are both 2nd messengers)
* IP3 works on ER to release calcium
* DAG and calcium combine with protein kinase C (PKC)
* conformational change in PKC activates it
* PKC phosphorylates a variety of membrane and cytoplasmic substrates
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Examples of enzyme-coupled receptors
* Tyrosine kinases
* JAK-STAT Receptors
* Serine/threonine kinases
* (note all create docking sites for other proteins)
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Receptor Tyrosine Kinases (RTKs)
* Enzymatic domain is in the cytoplasmic tail of the integral membrane protein
* Extracellular domain
* Transmembrane domain (single pass)
* Ligand binding to this receptor causes a conformational change
* induces dimerization of two receptor monomers
* Autophosphorylation occurs
* autophosphorylation causes the receptor to act as a scaffold to recruit other proteins to the plasma membrane
* Outside event (binding to receptor) gets transduced to a response inside the cell
* Receptor does not bind to G protein but receptor binds to proteins with domains called the SH2 domains (src homology)
* SH2 domain binds to phophotyrosine
* In mammals the SH2 protein is Grb2 (adaptor protein)
* RTK Importance
* Receptor tyrosine kinases are used for response to growth factors: mediate growth factor signals
* Growth factors are proteins released by cells to promote growth of other cells
* History:
* Grow tissue cells in culture and add amino acids, sugars and other stuff= no growth
* Cells are in contact indirectly with blood so add back serum factors (e.g. bovine serum) and result is growth
* Biomedical scientists then identified the growth factors in serum
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Receptor Tyrosine Kinase Activity
* Growth factor binding to receptor leading to diminerazation of RTK monomers and autophosphorylation
* RTK binds to SH2 domain of Grb2
* SH3 of Grb2 binds to **prolines** in SOS (son of sevenless)
* sevenless= controls phoreceptor development, receptor tyrosine kinase
* Ligand= BOSS (bride of Son of Sevenless)
* Downstream effectors found were Grb2 and SOS
* SOS binds to Ras (small monomeric G protein-small GTPase)
* Ras- first discovered human oncogene: plays crucial role in cell division and a frequent mutation in cancer
* Ras binds to Raf
47
Examples of growth factors
* EGF- Epidermal growth factor (53 aa's long)
* PDGF- platelet derived growth factor
* FGF- Fibroblast growth factor
* IGF-1-Insulin-like growth factor 1
* NGF- Nerve growth factor
* fxn: cause cells to grow and proliferate in cell culture
* (note that growth factor names were based on where discovered but these grwoth factors are found in multiple cell types)
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Example of RAS-dependent and RAS-independent signaling via RTK
Insulin signaling
50
Many signaling molecules are proto-oncogenes that can mutate into _____ and cause cancer
oncogenes
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JAK-STAT receptors
* More direct route for impacting transcription
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Serine-threonine receptors
* Smads can control cell proliferation
