1-38/39 Receptor families and signaling

Question 1

Receptor types fast to slow

Answer 1

Rapid Brief

• Ligand gated ion channels: aka ionotropic receptors
• GPCRS
• Transmembrane enzyme receptor (tyrosine kinases / JAK/STATs)**
• Cytoplasmic and nuclear receptors**

Slow sustained

**Alter gene behavior**

Question 2

Ligand-gated ion channels

Answer 2

• AKA ionotropic receptors
• Excitatory receptor channels-agonists open non-selective cation channels with the Na+ entry exciting the cell: usually it is Na+ entering but it could be other cations, but Na+ moves Vm more positive to depol
  
  1. Acetylcholine - nicotinic receptors
  2. Serotonin - 5HT-3 receptors
  3. Glutamate - AMPA/Kainate and NMDA receptors
  4. ATP - P2X receptors
• Inhibitory receptor channels-agonists selectively open Cl- channels: hyperpolarizes cell and makes less likely for AP to be able to fire
  
  1. Gamma-aino butyric acid (GABA) - GABAa receptors
  2. Glycine receptors

Question 3

CYS-loop receptors

Answer 3

named because alpha subunits (sites of agonist binding) have loop of cysteins, 4 membrane spanning domains

1. Ach nicotinic
2. Serotonin 5HT3
3. GABAa
4. Glycine

Question 4

Nicotinic Ach Receptors

Answer 4

• CYS-loop, excitatory
• initiate skeletal muscle contration, ganglionic transmission, excitatory stuff in CNS

Question 5

Serotonin 5HT-3 Receptors

Answer 5

• CYS-loop, excitatory
• concentrated in CNS sites associated with the emetic response
• Drugs in use: Ondan_setron_ (Zofran), competitive inhibitiors at 5HT3 receptors used to tx nausea vomitting (esp in cancer pts)

Question 6

GABAa Receptors

Answer 6

• Inhibitory, CYS-loop
• main inhibitory receptor in CNS, uses GABA as Neurotrans
• ligand-gated Cl- channel
• Drugs:
  
  • Benzodiazepines: azepam or azolam
    
    • ​7 membered ring with 2 nitrogens=benzodiazepine ring
    • positive allosteric modulator of the GABAa receptor
    • treat anxiety, produce sleep, reduce muscle spasms
  • ​Zolpidem (Ambien): sleep aid, not a benzodiazepine structurally but acts selectively at a subset of benzo binding sites
  • Penicillin: major side effect by targeting the GABAa receptor and blocks the Cl- channel once the channel is opened by GABA produces excitation: seizures, uncompetitive inhibitor

Question 7

Benzodiazepines

Answer 7

• Benzodiazepines: azepam or azolam
• ​7 membered ring with 2 nitrogens=benzodiazepine ring
• positive allosteric modulator of the GABAa receptor
• treat anxiety, produce sleep, reduce muscle spasms

Question 8

Glycine receptor

Answer 8

• CYS loops, inhibitory
• major inhibitory transmitter in spinal cord and only NT that acts exclusively on ligad-gated channels
• Drugs
  
  • Strychnine: competitive inhibitor of glycine recptors, causing excessive spasticity
  • Tetanus toxin: blocks glycine relase causing excessive spasticity
• Disease: familial startle disease of hyperekplexia
  
  • due to mutaitons in the alpha subunit of glycine receptor

Question 9

Penicilin vs. GABAa receptors

Answer 9

• Penicillin: major side effect by targeting the GABAa receptor
• and blocks the Cl- channel once the channel is opened by GABA produces excitation: seizures
• uncompetitive inhibitor

Question 10

Non-CYS loop receptors

Answer 10

1. Glutamate: major excitatory NT in CNS
   
   • ligand-gated ion channels two groups: NMDA receptors and non-NMDA (AMPA and kainate)
     
     • AMPA receptors: fast synaptic tranmission in CNS cause depolarization vai Na+ entry thru non-selective cation channels opened by glutamate
     • NMDA receptors: glutamate-gated “calcium” channels (other cations can also permeate), mediate learning and memory when normal levels of Ca+2 enter the cell, can cause excitotoxicity implicated in alzheier’s disease when excess ca+2 enters cell, responsive to glutamate and depolarization Mg+2 is stuck in channel so depol by some other channel is needed to kick out Mg+2, needs glycine to be fully activated
2. P2X receptors:
   
   • appear early in vertebrate development
   • invovled in every stage of wound healing
   • over abundamnt in diseased bladder, cause of overactive bladder
   • binds ATP and then allows ion flow

Question 11

NMDA receptors

Answer 11

• non-cys loop, glutamate-gated “calcium” channels (other cations can also permeate)
• mediate learning and memory when normal levels of Ca+2 enter the cell
  
  • can cause excitotoxicity implicated in alzheier’s disease when excess ca+2 enters cell
• responsive to glutamate and depolarization BUT Mg+2 is stuck in channel so depol by some other channel is needed to kick out Mg+2
• needs glycine to be fully activated
• Drugs: memantine (Namenda) open channel blocker of NMDA receptors used to tx Alzhemier’s

Question 12

hyperekplexia

Answer 12

Question 13

AMPA receptor

Answer 13

• NON-cys loop family
• fast synaptic transmission in the CNS casues depol via Na+ entry thru non selective cation channels
• opened by glutamate

Question 14

P2X receptor

Answer 14

ATP release:

• ATP is primitive NT
• ATP is stored innerve endinds with Ach and catecholamines and is relased all together when nerves are excited
• ATP and ADP are released drom damaged tissues

P2X ATP receptors:

• appear early in vertebrate development
• invovled in every stage of wound healing
• over abundamnt in diseased bladder, cause of overactive bladder

Question 15

Glutamate family receptors

Answer 15

Glutamate: major excitatory NT in CNSligand-gated ion channels two groups: NMDA receptors and non-NMDA (AMPA and kainate)

AMPA receptors: fast synaptic tranmission in CNS cause depolarization vai Na+ entry thru non-selective cation channels opened by glutamate

NMDA receptors: glutamate-gated “calcium” channels (other cations can also permeate), mediate learning and memory when normal levels of Ca+2 enter the cell, can cause excitotoxicity implicated in alzheier’s disease when excess ca+2 enters cell, responsive to glutamate and depolarization Mg+2 is stuck in channel so depol by some other channel is needed to kick out Mg+2, needs glycine to be fully activated

Question 16

GPCRs general

Answer 16

• largest family of proteins targeting drugs in clinical use
• metabotropic receptors- because act via secondary/tertiary messenger sunstances NOT ion channels
  
  • Inhibitory via ion channels opening
  • synthesis of second messengers
  • phosphorylation
• 2 components in GPCR signaling
  
  • THE RECEPTOR: changes conformation after binding of agonist which in turn activates the receptor partner (G-protein) (exchange of GDP bound to alpha subunit for GTP
  • THE GTP-BINDING PROTEIN: 3 subunits
    
    • Alpha: GDP bound (inactive), once exhanged for GTP (active) dissociated from beta/gamma subunits and initiate biological responses (Phospholipase C, adenylyl cyclase, generate synth of secondary messengers)
    • Beta/Gamma:linked, usually inhibitory of cell function

Question 17

GPCR: inhibitory motif

Answer 17

• Mediated by Gbeta/gamma subunits
• mechanisms include: activating K+ channels, inhibiting Ca+2 channels, decreasing cyclic AMP levels
• receptors often have subscript 2
• second fastest binding motif
• membrane-delimited process

Question 18

GPCR: smooth muscle contraction

Answer 18

1. agonist binds GPCR
2. conformational change, G-alpha-GDP exchange to G-alph-GTP
3. G-alpha-GTP activates PLC
4. PLC using PIP2 as substrate produces water soluble (IP3) and lipid soluble products
5. IP3 causes Ca+2 release from the ER and smooth muscle contraction

Receptors that signal like this often have odd number sucsripts

Question 19

GPCR: mediated phosphorylation

Answer 19

• PKA pathway: via activating from Galpha-S
  
  • ex: Beta adrenergic receptors activated and Gs activated adenylyl cylcase and ATP is converted into cAMP, cAMP activates PKA which phosphorylates substrates to produce biological effects
    
    • phosphatases and phosphodiesterases significant in response
• PKC pathway: via activating from Galpha-Q
  
  • ​ex: GalphaQ activates PLC which,
    
    • water soluble: synthesizes IP3 from PIP2 and evokes Ca+2 relases
    • lipid soluble: syntehsizes DAG from PIP2 which activates PKC, PKC activation then elads to phosphrylation of targets and can enhance the water soluble path

Question 20

GPCR: metabotropic glutamate receptors and GABAb receptors

Answer 20

look like a venus flytrap

Question 21

Agonist / Ion channel / GPCR

• ACH
• 5HT
• GABA
• Glycine
• Glutamate
• ATP
• Catecholamine
• Adenosine
• Opioid

Answer 21

Question 22

Transmembrane enzyme receptors (overview)

Answer 22

• require dimerizes confirugation beore signaling can ensue
• Receptor tyrosine kinases (RTKs): Growth factors
• After dimerization and reciprocal phosphorylation, P scaffold associations to initiate Kinase Kinase Kinase reactions which will result in alertered gene behavior and biological response

Types

1. PI-3-K/AKT/mTOR (Insulin)
2. RAS-MAPK (Growth factors)
3. JAK-STAT (interleukins)

Question 23

Growth factor receptors

Answer 23

• are Recetpro Tyrosine Kinases
• EGF, TGF-alpha, (released from cell types involved in wound healing) PDGF, FGF, VEGF
• WORK THROUGH MAPKS
• EGF: helpful to heal wounds, harmful to metastisize cancers

Question 24

Insulin receptor

Answer 24

• Insulin released from pancreatic beta cells in response to excessive gluce on blood stream
• Insulin bind RTKs on other cells to promote storage of glucose/fuels
• Insulin recpetors are present in a dimerized form in the absence of insulin-agonists

1. insulin binds receptor
2. kinases reciprocal phosphorylate
3. scaffold recruited (mediates RAS/MAPKinase pathway-> cell proliferation)
4. other scaffold, Insulin Receptor Substrate recruited ->PI3K->AKT->mTOR->mRNA/protein translated->glucose uptake/glycogen synthesis/fat storage

Question 25

Cytokine signaling

Answer 25

bind to transmembrane receptors, without tyrosine kinases of their own, that are linked in the cytoplasm via **JAK/STAT pathway** JAK: cytoplasmic kinase STAT: signal transducer and activator of transcription, bind to phosphohrylated JAK and gets phosphorylated, dimerizes and enters nucleus to alter gene transcription and coordinate immune response

Question 26

Cytoplasmic/Nuclear Receptor signaling

Answer 26

1. Steroids / Glucocorticoids * steroid can pass through cell membrane to bind glucocoritcoid receptor in cytoplasm * dimerize of steroid-glucocoritocid receptor * binds DNA in nucleus to change gene transcription * Inhibits inflmmatory cells, inhibits syn/release of inflam mediators 2. Environmental contaminantes/ herbal remedies (eg. st. john's wort) * bind cytoplasmic receptor, binds DNA, results in enzyme induction, induce drug metabolizing enzymes, SIDE AFFECTS

Question 27

Glucocorticoid receptor

Answer 27

* cytoplasmic, slowest of receptor types, most sustained response * bound by steroids * steroid can pass through cell membrane to bind glucocoritcoid receptor in cytoplasm * dimerize of steroid-glucocoritocid receptor * binds DNA in nucleus to change gene transcription * Inhibits inflmmatory cells, inhibits syn/release of inflam mediators

Question 28

Tachyphylaxis

Answer 28

tachyphylaxis: responses to acute repeated injections delcines when the interval is shortened continuous drug application: receptor desensitization * ex: continuous GABA or Ach application, ionic current plateaus off because there is changing of subunit confirmation of ligand gated channels (100 ms) * ex: continous Glutamate on AMPA receptors: desensitizes over the tiem of their normal activation (2 ms) * ex: GPCRs: 10 minutes desensitization, receptor degredation from the membrane -\> lysosome

Question 29

GPCR agonist desensitization scheme

Answer 29

repeated or prolonged dosing, receptors move to lysosomes and are downregulated and now resensitized

Question 30

Receptor Tyrosine Kinases agonist desensitization scheme

Answer 30

quick response, quick desensitize (2 hoursish) via internalization of receptors

Question 31

Insulin receptor desensitization scheme

Answer 31

half max response within 8-16 hours via receptor internalization

Question 32

JAK/STAT receptor desensitization scheme

Answer 32

half max wthin 4-8 hours from cytokine signaling, via receptor internalization

Question 33

glucocorticoid receptor desensitization scheme

Answer 33

half max wthin 4-8 hours from cytokine signaling, via receptor internalization

Question 34

competitive antagonists effect on receptors

Answer 34

* continual use leads to upregulation of receptors * discontinuuation of the drug can lead to rebound effects * particularly noteworthy with drugs that antagonize GPCRs

Question 35

allosteric activators effect on receptors

Answer 35

* frequently will **NOT stimulate the receptor by itself** * spatial/temporal harmony with natural agonist and less side effects likely * **reduced likelihoodof receptor desensitization** * ​ex: Zolpidem (Ambien) allosteric activator of GABAa can be prescribed for sleep aid for years

Question 36

Biased ligands at GPCRs

Answer 36

* Specific ligands can bias their effects to activate GPCR signaling over arrestin or vice versa instead of doing both * B-arrestin responsible for desensitzation * biased ligands change conformation of GPCR so diff phosphory patterns via GRKs occur, "bar code hypothesis", different GRKs establish unique phosphoryl barcosd which in turn recuti beta arrestins in distint conformation to bias the downstream response in one direction or another 1. **Morphine** acts on **Mu Opiod Receptors (MORs)**, _beta arrestin desensisitzes_ and is bad and gives all the side effects 2. Cardiovascular effects of angiotensin 2 receptors via AT1R, **-sartans are unbiased ligands**, b arrestin important drug target for therapy 3. cardioprotective effects of beta blocking drugs, EPI, NOREPI signal onto Beta-1, **carvedilol, beta blocker**, beta arresin mediates cardoprotection

Question 37

Barcode hypothesis

Answer 37

different GPCR ligands induce diff confomrational changes so that different GRKs are activated and give unique downstream phosplhrylation and activating, bias the direction of singaling in one direction or another