Receptor theory Flashcards
1
Q
By whom and when were ligand-gated ion channels first described?
A
J. N. Langley in 1905 first wrote that since muscle conctraction inhibitors do not prevent contraction via direct muscle stimulation, there must be an accessory substance they act through.
1
Q
What happens at neuromuscular junctions?
A
Acetylcholine binds to nicotinic receptors, allowing Na+ to flow through and K+ to flow out. Na+ ions dominate, leading to depolarisation of the muscle fibre.
This triggers an AP which travels along the t-tubules ultimately leading to the release of calcium ions from the sarcoplasmic reticulum (SR).
The released calcium ions bind to troponin, causing a conformational change and allowing the myosin and actin filaments to slide past each other and cause muscle contraction.
2
Q
When was the activity of nicotonic receptors first recorded and how?
A
Hamill et al (1981) recorded the activity of the receptors by fillimg a patch pipette with ACh and sealing it on the membrane
Were able to record a movement of ions into the cell
3
Q
What is the other name for ligand gated ion channels and what sort of transmission do they mediate?
A
Ionotropic receptors
Mediate fast synaptic transmission in the CNS and PNS
4
Q
What are the different families of ligand gated ion channels, what is there neurotransmitter, where are they located and how many subunits does each receptor have?
A
Pentameric, tetrameric and and trimeric
5
Q
What is the resting membrane potential of across the cell membrane?
A
~ 70 mV
6
Q
What is the balance of Na, Cl- and K+ ions in and out of the cell?
A
K+ = 140mM inside 3mM outside
Reversed for Na+
High levels of Cl- outside cell and lower inside
7
Q
What are inhibitory and excitatory ligand-gated ion channel examples?
A
Excitatory= Nicotinic (Na+, K+), 5-HT3 (Na+, K+), AMPA, (Na+, K+) and NMDA (Na+, K+, Ca2+)
Inhibitory= GABAa (Cl-) and glycine (Cl-)
8
Q
What is the structure of the nicotinic acetylcholine receptor?
A
It is pentameric with 4 transmembrane domains
M2 (green)= is the transmembrane section
Extracelluarly located NH3+ and COO-
9
Q
What are the two main types of ACh nicotinic receptor?
A
The muscle type at the NMJ
The neuronal type in the autonomic ganglia
They have completely seperate subunits
10
Q
What is homology like between the subunits of nicotinic ACh receptors?
A
- Homology between the same subunit across different species is very high (e.g. rat and human there is 90% homology)
- Homology between different subunits in the same species is lower (30-60%).
11
Q
Where is the ACh binding site on nictonic ACh receptors located?
A
ACh binding site is on the α-subunit + either γ- or δ-subunit; not exclusively contained in 1 subunit but at the interface.
12
Q
What type of charge lines the channel pore of nicotinic ACh receptors?
A
Rings of negative charge- attracts cations through the receptor
13
Q
What happens when ACh binds to nicotinic receptors?
A
There is a confirmational change and twisting of the TM2 to open the channel
14
Q
What are the subunits of nicotinic ACh receptors (muscular) and how does this change throughout development?
A
The subunits are designated as α (alpha), β (beta), γ (gamma), δ (delta), and ε (epsilon).
Whereas expression of the γ-subunit predominates in embryonic muscle, expression of the ε-subunit occurs during early neonatal development and persists in adults
15
Q
How does the depolrisation due to nicotinic ACh receptors differ between foetal and adult muscle?
A
Adult nAChRs have higher single channel conductances but shorter open times compared to the foetal form of the receptor
May need prolonged depolarisation in younger tissue to trigger an action potential
16
Q
What is an example of a drug that can be given for NMJ dysfunction?
A
Cholinesterase inhibitors- inhibits the enzyme that breaks down ACh and is currently being used in AD and myasthenia gravis (auto-immune disease)
17
Q
What protein has been used as a functional mimic for the ligand binding domain of nicotinic ACh receptors?
A
Acetylcholine binding protein (AChBP)
Found in certain molluscs such as snails and shares structural similarities with the ligand binding domain of nAChRs in vertebrates
It has been widely used as a surrogate for studying ligand-receptor interactions
It also serves as a model for screening and designing drugs that modulate the activity of these receptors
18
Q
What are the different subunits in neuronal and muscle nicotinic ACh receptors?
A
- Neuronal subunits= alpha 2-10 and beta 2-4
- Muscle subunits alpha-1, beta-1, gamma (foetal), epsilon, and omicron (adult)
18
Q
How do neuronal nictonic ACh receptor have different number of binding sites?
A
Made up of different combinations of alpha (2-10) and beta (2-4) subunits
5 alpha-7 subunits can form a receptor, meaning there is 5 binding sites.
There is different combinations in the CNS, PNS and immune system
19
Q
What conditions have agonists of nicotinic ACh receptors been trialled for?
A
Schizophrenia, AD, ADHD (attention deficit hyperactivity disorder), nicotinic addiction, pain and Parkinson’s disease
Autism, depression, cognitive deficits and psychosis
20
Q
How successful have nicotinic ACh receptor agonists been for treatment of neurology conditions?
A
Largerly failures of nAChR ligands in AD and schizophrenia in clinical trials have reduced enthusiasm for this therapeutic strategy and many pharmaceutical companies have now abandoned this field of research.
However, there is still nAChR modulating drugs in clinical trials
21
Q
What are examples of successful nictonic ACh modulating drugs?
A
Galantamine, a acetylcholinesterase inhibitor and positive allosteric modulator indicated for AD. It only reduces symptoms however, it does not cure it.
22
Q
What are the 4 receptor superfamilies and how long do they take to exert changes?
A
- LGICs/ ionotrophic receptors e.g. nicotinc AChR. Very fast reaction- ms
- GPCRs e.g. muscarinic AChR. Fast reaction- seconds
- Kinase linked receptors e.g. insulin or cytokines. Hours to activate
- Nuclear receptors e.g. oestrogen or cortisol. Activation takes hours
23
Where are nuclear receptors located?
Intracellularly (although are some caveats)
24
What activates nuclear receptors?
Activated by steroid hormones: oestrogen (ER), testosterone (AR), cortisol (GR), aldosterone (MR), progesterone (PR) as well as thyroid hormone (TR), vitamin D (VDR)
25
How does a nuclear receptor exert its changes?
They are ligand-activated transcription factors
Once a ligand binds, the receptor translocates to the nucleus where RNA polymerase acts.
Cause gene expression modulation
26
What are the main domains of a nuclear receptor?
27
What domain has a high level of homology between different receptors?
DNA binding domain
28
What is the role of the regulatory domain in nuclear receptors?
- Regulatory domain= very variable between receptors, both in sequence and length. It contains an important transcriptional activation function AF-1.
- AF-1 binds elements (co-activator or co-repressor proteins) that modify how the receptor interacts with transcription machinery- to silence or start transcription
29
What is the role of the DNA binding domain
Highly conserved between receptors, it localises on DNA and is involved in dimerisation
30
What does the DNA binding domain contain and how does this help its function?
It lcontains 2 Zn2+ fingers that wrap around the DNA helix
The C1 figer is involved in recognising specific DNA sequences to bind to
For example, the GR homodimer contains 4 zinc fingers which holds the protein in the correct shape to interact with the DNA
31
What is dimerisation?
Two receptors binding togther
Can be a heterodimer or a homodimer
32
How do nuclear receptors dimerise to each other?
Two receptors bind in mirror to each other
Some receptors (class 2 receptors) bind not in mirror but as 'spooning' each other. This includes the retinoid X receptor, thyroid hormone receptor and Vit D receptor
33
What is the function of the hinge domain in nuclear receptors?
A flexible hinge that links the ligand binding domain to the DNA binding domain and gives the receptor conformational flexibility
Has a role in dimerisation and allowing the receptor to find the nucleus
34
What is the role of the ligand binding domain in nuclear receptors?
The ligand typically binds within the ligand-binding pocket of the LBD, inducing structural alterations that expose the AF-2 domain.
The exposed AF-2 domain serves as a docking site for coactivator proteins which helps to enhance gene transcription
35
Where and what is the AF-2 region in nuclear receptors?
Sits in helix 12 of the ligand binding domain
Agonist ligands stabilise the receptor conformation that is optimal for efficient interaction with co-activators, with AF-2 domain exposed. Causes increased gene transcription
Antagonists have bulky side chains that hinder H12 from aligning in the agonist conformation.
36
What are the two classes of nuclear receptors?
- Class I
Endocrine ligands
High affinity
Mainly homodimers (although ERa/ERb; oestrogen receptor has two forms)
Cytosolic location (float around in the cytosol)
e.g. Steroid receptors: MR, GR, ER, AR, PR
- Class 2
Lipid ligands
Low affinity
Mainly heterodimer with RXR (except RXR)
Nuclear location- bound to DNA in the nucleus in an inactivated state (until ligand binds)
e.g. PPAR, LXR, RAR
- Also get hybrids
These have endocrine ligands, that heterodimerise with RXR. Include TR (thyroid) and VDR (Vit D)
37
What happens when a ligand binds to a nuclear receptor
In the AF-2 region, when not activated, the receptor is bound to co-repressor proteins.
Once ligand binds, co activator proteins bind and there is an opening up (acetylation) of the chromatin structure (or closing)
38
What are the different co-activator and co-repressor proteins that influence nuclear receptors?
- Co-activators: recruited by DNA bound, ligand bound receptor. Agonist ligands stabilise the receptor in a conformation that is optimal for efficient interaction with co-activators and facilitates transcriptional activation.
- Co-repressor: recruited by DNA bound unliganded receptor or antagonist bound receptor help repress transcription
- Different cohorts of co-activator and co-repressor proteins are expressed in a tissue specific manner
39
What is an example of a drug that acts as a ligand for nuclear receptors and acts in more than one way?
Glucocorticoids are used in asthma inhalers and for arthritis. They bind to the glucocorticoid receptor.
- Classical regulation pathway= GRs bind to the regulatory region of the gene encoding the anti-inflammatory protein annexin 1 (lipocortin) and increases its synthesis
- Alternative pathway= GRs repress COX and iNOS formation by binding to the pro-inflammatory transcription factors AP-1 and NFkB (which normally drive inflammation, and inhibiting their ability to promote gene transcription.
40
What is an example of a selective nuclear receptor modulator drug?
SERMS- selective oestrogen receptor modulators e.g. raloxifene
Mimic oestrogen in some tissues but antagonises it in others.
It is an agonist in bone and lipids (decreases LDL) but an antagonist on breast and endometrium
Can also get SARMS (selective androgen receptor modulators) that mimic testosterone in selected tissues only.
Useful as increases bone integrity and muscle strength without driving prostate cancers.
41
What evidence is there for non-genomic signalling from steroid receptors?
- Classical nuclear receptor signalling takes hours to transduce signal
- But also get very rapid effects of some steroids e.g. effect of oestrogen on blood vessels
- This only takes minutes and thus is not due to protein transcription.
42
What is the model of oestrogen action in endothelial cells?
The ER can be coupled with scaffolding to hold it to the cell membrane.
The receptor can then act clasically, or like a g protein coupled receptor
43
What do class II nuclear receptors dimerise with?
RXR (Retinoid receptor) which strengths DNA binding and transcriptional activity
44
What are the two types of class II nuclear receptor dimerisation?
Can be permissive (the complex can be activated by either RXR or the partner ligand) or non-permissive (the complex can only be activated by the partner ligand)
45
What year was the first nuclear receptor cloned?
1985
46
What are orphan nuclear receptors?
An orphan receptor whos endogenous ligand has not yet been identified
Orphan nuclear receptors are largely receptors for lipids: cholesterol (and metabolites), fatty acids and bile acids.
If the ligand is identified, the receptor becomes an adopted orphan
47
What are class II nuclear receptors mainly involved in?
Mainly involved in metabolism e.g. lipid profile in blood
Abnormal function leads to different blood lipid profile, insulin sensitivity, glucose tolerance, inflammation and adiposity
48
What are PPARs?
Peroxisome proliferator activated receptor
Activation reduces triglyceride levels
All PPARs herterodimerise with RXR
Endogenous ligands include free fatty acids and Vit B3- no domiant ligand however
49
What drugs target PPARs and what could be improved by them?
Clofibrate= ligand. Promotes fat efflux from tissue to liver for excretion. Lipid lowering agent
Glitazones= partial ligand of the PPAR-gamma receptor. Oral anti-biotic drugs designed to treat patients with type 2 diabetes
There is side effects of activating PPAR-gamma such as increased Na+ and water retention
50
How is pharma cuurently working to develop therapeutics targeting nuclear receptors?
- Pharma currently working to develop a series of ligands that result in different conformational transitions. Different ligands can differentially regulate gene expression by the same receptor by inducing selective co-activator/repressor binding
- This can be used to exploit the good effects of nuclear receptor signalling whilst not causing the bad.
51
What are liver X receptors and what are the potential therapeutic benefits?
LXRs are cholesterol sensors and regulate cholesterol homeostasis
They are activated by sterols such as oxysterol which are intermediates of cholesterol synthesis
Drug development pathway for cholesterol lowering, Parkinson's, cancer and inflammation
52
What are the subunits of ligand gated ion channels made up of?
4 membrane spanning helices
A pentameric receptor will contain 20 membrane spanning helices
53
How are the M2 subunits in ligand gated channels at rest and once opened?
Sharply kinked inwards at rest, forming a constriction
Once the channel is activated, a confirmational change occurs in the extracellular part of the receptor, causing the kinked M2 segments to swivel out of the way
54
How many molecules of ACh are needed to open the nAChR?
2- both binding sites need to be occupied for the receptor to open
55
What may the function of orphan receptors be?
One possible explaination may be their 'promiscuous' ability to bind to many related compounds with low affinity
56
What percentage of prescription drugs target nuclear receptors?
10-15%
57
Who first proved that chemical structures were important for substances to act?
Professor Arthur Cushney (chair of Edinburgh pharmacology) proved one isomer of adrenalin was more active than the other
58
What is the structure of GPCRs?
7 transmembrane alpha-helices
59
What is the structure of G proteins?
Trimeric- 3 different subunits (alpha, beta gamma).
60
What happens when a signalling molecule binds at GPCRs?
The alpha subunit exchanges GDP for GTP and dissociates
The alpha subunit and the beta-gamma dimer target anothe protein in the membrane and alter/regulate protein function
Target proteins can be channels or enzymes
61
How long do G protein effects last?
As long as the ligand is bound to GPCR, the effects can happen repeatedly
62
What is the RGS protein
Regulation of G protein signalling protein can speed GTP being hydrolysed to GDP
63
When were G proteins first isolated?
1977- Ross and Gillman
64
Why are GPCRs important in pharmacy?
~35% of all drugs approved (> 700 drugs) for use in humans target 134 GPCRs
65
Where do ligands and G proteins bind to?
Ligand binding= localised to the transmembrane domains
G protein= third cytoplasmic loop
66
What ligands are GPCR activated by?
Photons, hormones, N terminal protelytic cleavage
Not always ligands
67
How homologous are the subunits of the G protein?
Many different G-alpha subunits expressed in different areas of the body
There is less divisions in the beta subunits and the gamma subunit also has alot of variations
68
What happens when a ligand binds to the GPCR?
- When a ligand binds to the GPCR, it induces a conformational change that allows the G protein to bind to the receptor.
- The binding of the G protein to the activated receptor causes the alpha subunit to exchange its bound guanosine diphosphate (GDP) for guanosine triphosphate (GTP), leading to the dissociation of the alpha subunit from the beta-gamma complex.
- The activated alpha subunit (now bound to GTP) and the beta-gamma complex both regulate various intracellular signaling pathways and effector proteins. These pathways can involve the activation or inhibition of enzymes, ion channels, and other cellular effectors.
69
How are the effects of GPCRs terminated?
Can inactivate itself- To terminate the signal, the alpha subunit's intrinsic GTPase activity hydrolyzes GTP to GDP, leading to the reassociation of the alpha subunit with the beta-gamma complex.
70
What is a GEF?
Guanine nucleotide exchange factor
Facilitates the exchange of guanosine diphosphate (GDP) for GTP, leading to the activation of the G protein
71
How might RIC-8 act as a GEF?
It can induce partial G-alpha unfolding, thereby enchancing GDP release
72
What is an orthostatic binding site?
Natural ligand binding sites on a G protein coupled receptor for the endogenous ligand. These binding sites are also recognised by classical agonists, competitive antagonists and inverse agonists
73
What is a partial agonist?
An agonist that produces a signalling response that is lower than the maximum response achievable for the given signalling system. Can antagonise the effects of fully agonists
74
What is inverse agonists?
Ligands that bind to a receptor and decrease its basal signalling activity
75
What are allosteric binding sites?
Seperate from orthosteric sites, causes a change in the shape and activity of the receptor that results from combination with a substance not in the orthosteric site
76
What is useful to know about the about the 5HT receptor?
It is a GPCR
There is a weakness in the 5HT receptor structure beyond the binding site and ligands can interact there and change the structure- useful to know for drug discovery
77
What are the different types of allosteric ligands acting on GPCRs?
- Allosteric antagonists= Ligands that bind to the allosteric site but do not have efficacy, yet their binding disrupts or inhibits the binding of other allosteric ligands.
- Neutral allosteric ligands= (Previously referred to as silent allosteric modulators). Allosteric ligands that bind to a G protein-coupled receptor but do not alter the affinity or efficacy of the orthosteric ligand.
- Negative allosteric modulators= Allosteric modulators that decrease the affinity and/or responsiveness of the orthosteric ligand.
78
What is the parameter used to describe and quantify the intensity of the effect of allosteric ligand binding on the affinity of the orthosteric ligand?
Cooperativity factor alpha
79
What is the primary role of beta arrestins?
Desensitisation of GPCRs, preventing further interaction with G proteins
This process dampens the cellular response to continuous exposure to the ligand
Also facilitate the the endocytosis of the receptor, bringing it into the cell through vesicle formation. This helps regulate the number of receptors on the cell surface
80
How do beta-arrestins serve as signalling molecules
They can initiate intracellular signaling pathways independent of G proteins, and this has led to the recognition of "biased signaling"
These pathways can be distinct from traditional G protein-mediated pathways.
81
What is biased signalling?
There can be two signalling pathways within one receptor and an agonist can act quicker in one pathway- An agonist can have a higher efficacy in one pathway, but lower efficacy than a different agonist in a different pathway
82
What is a biased allosteric agonist?
It changes the confirmation of the receptor so only a specific pathway can activate it
83
How can biased allosteric agonists be used in pharmacology?
Can be used in drugs to stop unwanted pathways that lead to side effects in patients.
84
What is probe dependance?
Probe dependance= depending on the affinity and efficacy of the orthostatic ligand, the effects of the allosteric ligand will give a different outcome
85
What is a biased allosteric modulator?
Binds to allosteric site
Modulates different pathways- e.g. BAM may increase one pathway, may allow a pathway to occur when there is no ligand, decreases a pathway, allows only the beta arrestin signalling pathway
86
What are the different ways that GPCRs can cause signalling?
87
What is an example of a drug that exerts its effects by biased signalling in opiod GPCRs?
TRV130 causes less of the side effects that morphine dose- e.g. less constipation and respiratory depression at equianalgesic doses
This was tested via how long a rat could hold its foot on a hot plate for
88
How could biased signalling be useful in heart failure?
Clinical hypothesis= beta arrestin ligands would be useful at the angiotensin II type 1 receprtor
Preclinical data suggest that Gαq coupling at AT1R is linked to vasoconstriction and sodium and fluid retention, whereas β-arrestin-2 recruitment is associated with contractility and reduced apoptosis
89
What are some of the potential issues that arise when targeting a receptor during drug development and screening?
Metabolites= simple structure changes can drastically change how they act at receptors e.g. go from a NAM to a PAM
Different receptor subtypes= drugs may have different/ no effect at different receptor subtypes
Preclinical animal models= confirm that receptor dependent effects are unaltered in model species
90
What was the first GPCR to be cloned?
Beta adrenoceptor in 1986
91
What is a PAR?
Protease activated receptor
GPCR
Proteases such as thrombin activate PARs by snipping off the end of the N-terminus tail of the receptor
92
How does GPCR action terminate?
Attachment of the alpha subunit to an effector molecule increases the activity of GTPase, and so the reaction is usually self-limiting
There is also regulators of G protein signalling proteins
93
Can G proteins act on more than one receptor?
Yes- a single G protein can act on several receptors
The heterogenity of different alpha subunits is what allows G proteins cause different effects
94
What does G alpha s and i do?
s= Stimulates adenylyl cyclase, causing increased cAMP formation
i= inhibits adenylyl cylase, decreasing cAMP
95
What do beta-gamma G protein subunits act on?
Activate K+ channels
Inhibit voltage gated Ca2+ channels
Activate GPCR kinases
96
What are the downstream targets of GPCRs?
Inlcude signalling enzymes, adapter proteins, cytoskeletal and trafficking proteins, ion channels and transcription factors
97
How does signalling amplification occur in GPCR activation?
Each effector protein/enzyme can produce lots of secondary messengers and cause amplification
98
What does an increase in cAMP target?
PKA- protein kinase A
Has a role in ion channels, cytosketelton trafficking proteins and transcription factors
It regulates glycogen, glucose and lipid metabolism
99
What are examples of toxins that modulate GPCR signalling?
Cjolera toxin- switches on G alpha s and causes long-lasting action. Causes diarrhoea due to the massive increase in cAMP signalling that upregulates channels in the gut wall
Pertussis toxin- causes inactivation of G alpha i causing inactivation.
100
What is phosphorylation?
Phosphorylation is the addition of a small highly polar phosphate group using ATP and causes confirmational changes in proteins
101
What do phosphorylating enzymes do?
Phosphorylate different proteins, which activates, inhibits, exposes sites or disrupts sites for protein-protein interactions
Known as KINASES
102
What are protein phosphatases?
Phosphatase enzyme that removes a phosphate group from a phosphorylated protein
Reverses the action of kinases
103
What are AKAPs and what is their function?
AKAPs= A-kinase anchoring proteins
Act as scaffold proteins and bind PKA and physically tether these to subcellular locations.
By doing so, they ensure that PKA is in close proximity to its substrate proteins, allowing for efficient phosphorylation and regulation of downstream targets.
104
What is a specific example of a AKAP?
TRPV1 (a noxious heat sensitive channel) is regulated by PKA, which is scaffolded by AKAP79
105
How does GPCR desensitisation occur?
3 steps:
Phosphorylation of activated receptor by GPCR family kinases
Binding of arrestins to phosphorylated residues
Prevention of G protein access to the GPCR
106
What are arrestins?
A family of four proteins that regulate the signaling and trafficking of hundreds of different G-protein-coupled receptors (GPCRs).
107
Why would we want to stop GPCR desensitisation?
Stop the desensitisation of Beta-1 receptors in heart failure
Stop desensititsation of brain dopamine receptors in Parkinson's disease
GCPR kinase 2 inhibitor Takeda compound 101 was found to reverse desinsitisation of Mu opiod receptors
108
How could GPCR desensitisation be targted pharmacologically?
A potential strategy to rescue defective GPCR is to inhibit GRK/ arrestin mediated desensitisation
However, GRKs are used for many receptors and if a GRK inhibitor was given many receptors would fail to be desensitised and it is not good in practise.
109
How are arrestins involved in GPCR recycling?
Arrestin binds with AP-2 and clarthin which are found in clarthrin coated pits (destined to become endoscopes)
The endosome is either recycled or degraded
110
What major cellular processes do arrestins have a role in?
By acting as scaffolds that bind key pathway intermediates arrestins influence the level of pathway activity in cells.
As signaling scaffolds, arrestins are also central regulators of pathways controlling cell growth, migration, and survival, suggesting that manipulating their scaffolding functions may be beneficial in inflammatory diseases, fibrosis, and cancer.
111
What are the ionotropic glutamate receptors?
112
How many subunits do the ionotrophic glutamate receptors have?
4- they are tetrameric
113
What are the subunits of ionotropic glutamate receptors called?
AMPA= GluA1, GluA2-4
NMDA= GluN1, GluN2-A-D, GluN3A,B
Kainate= GluK1-5
114
What NMDA receptor subunits are unusual and why?
GluN3a and GluN3b
They can be incorporated into NMDA receptors but cannot bind glutamate
115
What are antagonists for AMPA and NMDA receptors?
AMPA= CNQX, NBQX and ATPO
NMDA= AP5/APV, Mg2+, MK-801, ifenprodil, H+
116
What is the structure of AMPA receptors?
AMPARs are composed of four subunits
The second transmembrane domain does not cross the membrane but kinks back on its and returns to the intacellular side
When the 4 subunits come togther, this domain forms the ion pore
Each AMPAR has 4 agonist binding sites
117
How fast do AMPARs open and close?
Quickly- 1ms
Responsible for fast excitatory synaptic transmission in the CNS
1000s of ions move per ms
118
When do AMPA receptors open?
When 2 of the binding sites are occupied, and its current increases as more binding sites are occupied
119
What ions can pass through AMPARs?
Ca2+, K+, Na+
If an AMPAR lacks a GluA2 subunit, then it will be permeable to sodium, potassium, and calcium. The presence of a GluA2 subunit will almost always render the channel impermeable to calcium.
120
Where in the CNS are AMPARs located?
Both presynaptic and postsynaptic membranes
121
When do NMDARs open?
NMDA receptors open following depolrisation of the postsynaptic (dendritic) spine by AMPA receptor activation
The influx of sodium causes an action potential. NMDAR usually blocked by Mg2+ sitting in the pore, however depolarisation causes the Mg2+ to leave.
- Ca2+, N+ and K+ flows through the NMDA receptor
122
Why is the GluA2 a major derminent of AMPAR properties?
RNA editing of this subunit results in an Arginine (R) residue being expressed at the apex of the M2 region of the protein although the gene codes for a Glutamine (Q) residue in all other subunits
Arginine carries a positive side chain and this positive charge repels the cations so decreases conduction.
Also makes it impermeable to Ca2+
123
How do the AMPAR subunits change with age and how does this affect current?
GluA2-lacking AMPARs are predominantly expressed in early development and GluA2 AMPARs increase with age
GluA2 lacking allow Ca2+ to move through
Therefore, there is a greater variability of current in young ages
124
What is a blocker of AMPARs that lack GluA2?
NASPM
1-naphthyl acetyl spermine (NASPM) blocks AMPAR-currents at Week 2 but not at Week 5
125
How does desensitisation of AMPARs occur?
Desensitisation occurs easily
The mechanism of desensitisation is believed to be due to a small change in angle of one of the parts of the binding site, closing the pore.
126
What is the most common inhertited cause of ALS?
C9ORF72 hexanuleotide repeat expansion
127
What is a hypothesis for how AMPA receptors are thought to be involved in ALS?
C9ORF72 MNs express more Ca2+ permeable AMPARs compared with controls
Variance of current is much larger. Excess glutamate causes influx of Ca2+ through GluA2 lacking channels causing toxicity; mitochondrial dysfunction and cell death
Proven by a study that proved C9 cells did not have a reduced conductance compared with control but did have increased blockage by NASPM
128
What is a potential therapeutic for ALS patients?
Less cell death in cells with C9ORF72 mutation with CNQX (anatagonist of AMPA) and JTX (blocks calcium permeable receptors)
129
What do NMDA receptors mediate?
The slow component of glutamatergic excitatory post-synaptic current
130
What processes are NMDARs involved in?
Long-term potentiation (LTP) and long-term depression (LTD), two forms of synaptic plasticity, involve the activation of NMDA receptors.
Important for mediating learning and memory
131
What drugs are antagonists of NMDARs?
- Phencyclidine (former vetinary anaesthetic)
- Alcohol
- Dextromethorphan (supresses dry cough)
- Ketamine and Nitrous oxide effects are partially due to NMDA antagonism
132
What subunit do all NMDARs contain?
All NMDARs contain one or more of the obligatory GluN1 subunits
These are required for he binding of glycine, which acts as a co-agonist
133
How could NMDARs be used to treat neurological conditions?
- Overactivation of NMDA receptors, causing excessive influx of Ca2+ can lead to excitotoxicity.
- Excitotoxicity is implied to be involved in some neurodegenerative disorders such as Alzheimer's disease, Parkinson's disease and Huntington's disease
- Blocking of NMDA receptors could therefore, in theory, be useful in treating such diseases.It is, however, important to preserve physiological NMDA receptor activity while trying to block its excessive, excitotoxic activity, possibly achieved by uncompetitive antagonists.
134
How are GluN2 NMDA subunits spatially expressed?
An in situ study of mRNAs encoding NMDA receptor subunits in the developing rat CNS revealed that, at all stages, the N1 gene is expressed in virtually all neurons, whereas the four N2 transcripts display distinct expression patterns.
135
What are the different domains of an NMDAR?
Amino-terminal domain
Ligand-binding domain
Mmebrane domains
Carboxy-terminal domain
136
What is the function of the amino-termina domain in NMDARs?
ATD= contains binding sites for regulatory compounds such as zinc ions.
137
What is the function of the ligand binding domain?
Have both glycine (GluN1 subunit) and glutamate binding sites. Binding of both glutamate and glycine is required for the activation of NMDA receptors
138
What is the function of the membrane domains in NMDARs?
Form the ion pore and are involved in regulating ion flux, and the opening of the ion channel is dependent on glutamate and glycine binding.
139
What is the function of the c terminal in NMDARs?
Plays a role in intracellular signalling, trafficking of the receptor within the cell, and interactions with other proteins.
140
What pharmacological and biophysical properties do the N2 subunits of NMDARs determine?
2A receptors have low GluN1 and GluN2 agonist potency as well as fast deactivation rate. Also has higher Mg2+ sensitivity, higher Ca2+ permeability and higher conductance.
Opposite for 2D
141
How fast is the unbinding of glutamate from NMDARs and why is this the case?
- The duration of glutamatergic EPSCs indicated that the unbinding of glutamate is slow
- If block NMDARs with AP5, the EPSCs return to normal much quicker
142
What controls the differences in conductance between the GluN2 subunits of NMDARs?
A Ser/Leu site in M3 controls NMDAR single-channel conductance
It was found that differences in Mg(2+) block, selective permeability to Ca(2+) and single-channel conductance were all controlled primarily by the residue at a single GluN2 site in the M3 transmembrane region
This residue couples with the pore forming loop of the GluN1 subunit to create naturally occuring variations in NMDARs
