Lecture 18: Cell Communication

Question 1

Extracellular signaling molecules bind to specific receptors in target cells to initiate a chain of events referred to as

Answer 1

Signal transduction

Question 2

External signals induce what 2 major types of responses

Answer 2

• 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)

Question 3

____ is released from fat and signals hypothalamus that you are full

Answer 3

Leptin

Question 4

Types of cell signaling

Answer 4

• Endocrine signaling
• Paracrine signaling
• Autocrine signaling
• Direct Signaling

Question 5

Endocrine Signaling

Answer 5

• 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

Question 6

Paracrine signaling

Answer 6

• Acts locally
• Affects cells nearby (not as freely diffusible)
• Short lived signal
• e.g. neurotransmitters

Question 7

Autocrine Signaling

Answer 7

• 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

Question 8

Direct cell signaling

Answer 8

• also known as juxtacrine signaling
• e.g. immune cells
  
  • Ag-presenting cells to T cells

Question 9

Does each cell interpret the combination of all types of signaling to determine actions

Answer 9

Yes

Question 10

Examples of Acetylcholine having different effects on different types of cells (ex. of same ligand-different responses)

Answer 10

• relaxs heart muscle cells
• contracts skeletal muscle cells
• causes the secretion of saliva by salivary gland cells

Question 11

How does signal Transduction work

Answer 11

Question 12

what are the componets of the cell signaling cascade

Answer 12

• 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⁺)

Question 13

Cell Signaling: ligands

Answer 13

• 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

Question 14

Examples of Lypophilic (“lipid-loving”) signaling molecules (ligands)

Answer 14

• Steroid hormones: progesterone, estradiol, testosterone, cortisol, aldosterone, vitamin D
• Thyroid hormone: Thyroxine
• Retinoids: retinol, retinoic acid

Question 15

Receptor location and type for lypophilic (“lipid-loving”) ligands

Answer 15

• Found in the cytoplasm and nucleus
• Family of DNA-binding transcription factors

Question 16

Examples of hydrophilic (“water-loving”) signaling molecules (ligands)

Answer 16

• Amino acid derived:
  
  • histamine, serotonin, melatonin, dopamine, norepinephrine, epinephrine
• From lipid metabolism:
  
  • acetylcholine
• Polypeptides:
  
  • insulin, glucagon, cytokines, thyroid-stimualting hormone

Question 17

Receptor location and type for hydrophilic (“water-loving”) ligands

Answer 17

• Found on the surface of plasma membranes
• includes transmembrane proteins such as G protein-coupled receptors and receptor tyrosine kinases

Question 18

What are the two general types of receptors

Answer 18

• 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

Question 19

most signaling molecules are

Answer 19

hydrophilic and require cell-surface receptors

Question 20

Question 21

What are the three main types of cell signaling receptors in the Plasma membrane

Answer 21

• Ion-channel-coupled receptors
• G-Protein Coupled receptors
• Enzyme-coupled receptors

Question 22

G-protein coupled receptors use ___ pass transmembrane proteins

Answer 22

7

Question 23

whaat type of signaling recepotrs are common in nervous tissue

Answer 23

Gated ion

Question 24

______ receptors include receptor tyrosine kinases (RTKs)

Answer 24

Enzyme-coupled receptor class

Question 25

Transmembrane receptors

Answer 25

• 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)

Question 26

G-Protein coupled receptors

Answer 26

• 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 —-> 2^nd messenger —–> targets of 2nd messenger —-> biological process
• GPCR do not transfer phosphates (not active kinases)

Question 27

• 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.

Answer 27

• GDP is bound to the alpha subunit and the G protein is inactive

Question 29

The alpha subunit in GPCR is a GTPase and becomes inactive when

Answer 29

it hydrolyzes its bound GTP to GDP

Question 30

Steps of G-protein relaying signals

Answer 30

• 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

Question 31

Question 32

cAMP targets

Answer 32

• 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

Question 33

Question 34

Consequences of protein phosphorylation by PKA

Answer 34

• 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

Question 35

Question 36

What is the molecular mechanism by which cholera toxin acts

Answer 36

• 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

Question 37

The ability to turn off or reject the signal

Answer 37

Desensitization (this is a very important characteristic think about cancer)

Question 38

what are potentiate and attenuate

Answer 38

• Potentiate= turn up
• Attenuate= turn down

Question 39

Ways to attenuate or desensitize signal

Answer 39

• 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

Question 40

What are GRKs

Answer 40

• 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

Question 41

G_i/oalpha G-protens function

Answer 41

inhibit Adenylate cyclase and thus cAMP is not produced and PKA is not activated

Question 42

G_qalpha G-proteins function

Answer 42

• 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

Question 43

Question 44

Examples of enzyme-coupled receptors

Answer 44

• Tyrosine kinases
• JAK-STAT Receptors
• Serine/threonine kinases
• (note all create docking sites for other proteins)

Question 45

Receptor Tyrosine Kinases (RTKs)

Answer 45

• 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

Question 46

Receptor Tyrosine Kinase Activity

Answer 46

• 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

Question 47

Examples of growth factors

Answer 47

• 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)

Question 48

Question 49

Example of RAS-dependent and RAS-independent signaling via RTK

Answer 49

Insulin signaling

Question 50

Many signaling molecules are proto-oncogenes that can mutate into _____ and cause cancer

Answer 50

oncogenes

Question 51

JAK-STAT receptors

Answer 51

• More direct route for impacting transcription

Question 52

Serine-threonine receptors

Answer 52

• Smads can control cell proliferation