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Physiology: References & Stats > Cell Phys > Flashcards

Cell Phys Flashcards

1
Q

What are ABC transporters?

A
  • Transport system superfamily

- Largest protein family so far with 1-3% of genomes coding for their subunits

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2
Q

What is the general function of ABC transporters?

A

Unidirectional importing and exporting proteins against their chemical gradients
Eukaryotes- exporting

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3
Q

Describe the molecular structure of ABC transporters

A

Characterised by:

  • Two nucleotide binding domains (NBDs)
  • Two transmembrane domains (TMDs)

Also has:

  • Phosphate- binding loop (P-loop)
  • Short signature sequence (LSGGQ)
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4
Q

What have studies observed about the gating of ABC transporters?

A

Balarkrishnan et al. - drug efflux pump LmrA has been shown to be reversible in certain conditions.
This suggests that the membrane domain has ‘turnstile’ like gates rather than barriers.

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5
Q

What are the problems with the ATP switch model?

A

There is the general agreement that these steps must occur at some point but less understanding of the exact order.

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6
Q

List examples of human ABC transporters

A 
P-glycoprotein (p-gp)
MRP1
ABCA1
SUR1 (ABCC8)
CFTR
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7
Q

Describe the functions of p-glycoprotein

A
  • Transports neutral and cationic hydrophobic compounds
  • Transports xenobiotics out of cells
  • Exploited by tumour cells
  • Expressed in filter organs (e.g. liver, kidney, intestine…)
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8
Q

What does p-gp have to do with multi-drug resistance?

A

Expressed at high levels in tumours -> transports anticancer drugs out of cancer cells

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9
Q

How does p-gp know what the cytotoxic level of anti-cancer drug is?

A

1- Already a small population of p-gp which survives and replicates via natural selection during cancer treatment

2- All cells have small concentration of p-gp but the instability of the cell cycle means that the tumour cell can up regulate p-gp expression

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10
Q

What is the role of MRP1?

A

Transports anionic compounds

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11
Q

What is MRP1 named a ‘multi-drug resistance associated protein’?

A

It’s expressed in most tissues (and therefore present in cancers).

This means that like p-gp, it can export anticancer drugs out of the cell.

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12
Q

What is the role of ABCA1?

A

Cholesterol efflux from:

  • Macrophages
  • Placenta
  • Liver
  • Lungs
  • Adrenals
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13
Q

Describe the consequences of ABCA1 mutation

A

Tangier disease
Familial HDL deficiency, characterised by:
- Low HDL levels
- Lipid dense macrophage deposits in tissues
- Atherosclerosis and associated diseases

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14
Q

How does ABCA1 mutation cause familial HDL deficiency?

A
  • Usually, app A can strip lipids from the membrane via the ABCA1 receptor
  • ApoA can then get loaded, becoming a mature HDL particle which goes to the liver to get discarded.
  • Stopping the ABCA1 receptor stops cholesterol and lipid from leaving the cell so it accumulated.
  • Cells can get deposited anywhere - plaque formation more likely.
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15
Q

What is the role of SUR1?

A
  • Interaction with KATP channels

- Insulin secretion

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16
Q

Describe the structure of the SUR1 receptor

A
  • 4 SUR1 subunits

- Pore formed by 4 Kir6.2 subunits which potassium can move through when unblocked

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17
Q

How can the SUR1 channel be inhibited?

A
  • Sulfonylureas (e.g. metformin) to NBD

- ATP or ADP to the Kir6.2 subunit

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18
Q

How can the SUR1 channel be stimulated?

A

MgATP and MgADP binding to NBD1 and NBD2

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19
Q

How do we know how KATP channels behave in insulin secretion?

A
  • Calcium channels are always open whenever there is depolarisation
  • Where calcium channels are open, potassium channels must be closed and vice versa
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20
Q

Describe medical problems which affect SUR1

A

Hyperinsulinism – beta islet cell membranes are always depolarised (due to MgADP insensitivity) -> increased insulin release-> low glucose

Neonatal diabetes – beta islet cell is always hyperpolarised (due to ATP insensitivity) so insulin is never released -> high glucose

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21
Q

What is the function of the ABCC7 transporter?

A

CTFR- transport of chloride ions out of cells

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22
Q

How is the gating of the CFTR protein regulated?

A
  • Protein kinase A (PKA)- mediated phosphorylation
  • ATP interaction (Muallem 2009)
  • Nucleotide content of nucleotide binding domains (Wilkens 2015)
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23
Q

What is important for CFTR activation?

A

CFTR needs to be phosphorylated by PKA in the presence of ATP

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24
Q

What is the difference between SUR1 and CFTR?

A

SUR1 regulates electrical conductance, CFTR is just a chloride ion channel

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25
Q

What kinds of mutations occur to the CFTR protein?

A

Delta- alpha508 - channel partially active and stuck in endoplasmic reticulum so it can’t transport Cl- ions

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26
Q

How does the CFTR protein close?

A

Nucleotide of the dimer dissociates (likely due to instability) and the channel closes.

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27
Q

Outline the stages of CFTR transport with reference to the ATP switch model

A
  • ATP binds, channel opens so Cl- can travel through; inward movement of binding site
  • PKA phosphorylates, triggering outward movement of binding site
  • Complex becomes unstable
  • ADP association
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28
Q

What are the normal intracellular and extracellular concentrations of calcium?

A

Intracellular- 100nM

Extracellular- 2mM

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29
Q

Outline the underlying mechanism behind hyperalgesia

A

TRPV1 is under the influence of many messengers

Increased receptor function -> hyperalgesia

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30
Q

What is the difference between allodynia and hyperalgesia?

A

Allodynia - touch is associated with pain, provoking nociceptors -> neuroma formation

Hyperalgesia- pain stimulus is present but individuals have higher sensitivity

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31
Q

Outline the rationale behind functional cloning

A
  • Look at which receptor responds to capsaicin
  • Take the whole RNA for that receptor
  • Make a library
  • Express that library in a cell you know doesn’t normally express it
  • If the cell that normally doesn’t perform the function suddenly does after you add the library, you know it has to be down to the RNA that was added.
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32
Q

What are the response properties of nociceptors?

A

Adaptation- for a constant stimulus, action potentials slowly decrease
Fatigue- after adaptation to a constant stimulus, they need time to recover
Sensitisation- after injury, this is specific to heat and mechanical stimuli

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33
Q

What are the stages of encoding and processing noxious stimuli?

A

Transduction - detecting noxious stimuli
Conduction - transmission of information along the nerve fibres
Transmission - at the first pain synapse in the spinal cord

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34
Q

How is pain distinct from other sensory stimuli?

A
  • Receptor sensitivity

- Pain only becomes pain once nociceptors have interacted with the limbic system

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35
Q

Where does specialised detection of pain signals take place?

A

Upper lamina in the substantia gelatinosa

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36
Q

What are the constituents of ABC transporters in eukaryotes?

A

Single polypeptide with all four functional units

Some units assembled with ‘half’ transporters; can be homodimeric or heterodimeric.

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37
Q

What is the nucleotide binding domain otherwise referred to as?

A

ATP-binding cassette

i.e. the hallmark of the ABC transporter family

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38
Q

What is responsible for the identification of nucleotide binding domains of ABC transporters?

A

Signature sequence LSGGQ

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39
Q

Describe the properties of transmembrane domains

A
  • 6-10 transmembrane alpha-helices (most exporters have 6)

- Since some are dimers, that leads to a total of 12-20 respectively.

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40
Q

What is the difference between ‘inward’ and ‘outward’ facing transporters

A

Inward - pore accessible from cytoplasm

Outward - pore accessible extracellularly

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41
Q

Why has p-glycoprotein been described as having ‘polyspecificity’ towards its transport substrates?

A

It has several overlapping drug-binding sites.

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42
Q

How do ABC transporters move substrate with respect to their chemical gradients?

A

With few exceptions, ABC transporters pump them against their gradient.

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43
Q

Give an example of a eukaryotic ABC importer

A

Vitamin B12 uptake

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44
Q

With reference to the ATP switch model, describe how ABC exporters work

A
  1. Substrate binding to the transmembrane domain (TMDs)
  2. ATP molecules bind to the nucleotide binding domains (NBDs)
  3. NBD dimerises, TMD change in conformation (inward -> outward)
  4. ATP hydrolysis; NBD dissociation, phosphate, ADP and substrate release

Cycle then resets transporter to ground state

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45
Q

What is the proposed mechanism of ATP hydrolysis in the ATP switch model?

A

Base catalysis by glutamate residue at the end of a phosphate binding loop.

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46
Q

What is the ABCC1 transporter more commonly known as?

A

MRP1

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47
Q

What is the ABCB1 transporter more commonly known as?

A

P-gp

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48
Q

List the ABC transporters involved detoxification

A

ABCB1
ABCC1
ABCG2

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49
Q

What does ‘degenerate’ mean with respect to ABC transporters

A
  • SUR1 and CFTR both have one of these as their nucleotide binding domain.
  • They can still bind ATP but it isn’t hydrolysed as efficiently.
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50
Q

What is the ABCC8/9 transporter more commonly known as?

A

SUR

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51
Q

List the different types of carriers/ transporters

A

Uniporters
Cotransporters
Antiporters

52
Q

What is the difference between carriers and channels?

A

Only carriers carry out active transport

Carrier- alternates between two conformations so solute binging site alternates in accessibility

Channel - from water-filled pore across bilayer that’s ion specific

53
Q

Compare symporters and antiporters

A

Both examples of coupled transport
Symporters- transport molecules in same direction
Antiporters- transport molecules in opposite directions

54
Q

Why are carriers useful for diffusion?

A
  • Transport rate is higher
  • Saturable process
  • Kinetics are similar to enzyme catalysis
55
Q

What is the difference between ion channels and single aqueous pores?

A

Ion channels:

  • have a selectivity filter
  • aren’t always open- they’re gated
56
Q

What are the distributions of intracellular and extracellular sodium and potassium?

A

Sodium
Intracellular - 10mM
Extracellular - 140mM

Potassium
Intracellular - 145mM
Extracellular - 4mM

57
Q

What is the function of specialised channels and pumps?

A

Specialised channels conduct ions down their electrochemical gradients

Specialised pumps maintain ion gradients by pumping ions against their gradient

58
Q

How do you determine how substrate flux (movement) is mediated?

A

Generally speaking, channels have higher conductance levels than pumps though the distinction is not foolproof

59
Q

What is the difference between ion channels and ion exchange pumps?

A

Ion channels only need one gate which can be open/ closed at any one time

Ion exchange pumps need two gates which are never both open at the same time

60
Q

How do glutamate transporters behave?

A

Carrier/exchanger for glutamate, coupled with protons, sodium and potassium ions

Channel for chloride ions

61
Q

Describe the structure of a bacterial glutamate transporter

A

Trimer
Structural evidence suggests that each subunit binds its amino acid substrate and acts as a transporter/ chloride channel independent of the other monomers.

62
Q

Why are nociceptors ‘psuedounipolar’?

A

They don’t have dendrites

63
Q

Where are the majority of nociceptors found?

A

Dorsal root ganglion (peripheral nervous system)

64
Q

How are noxious stimuli detected?

A

Free nerve endings detect tissue damage and noxious signals via:

  • Chemicals
  • Temperature extremes
  • Mechanical insults
65
Q

Describe the cellular diversity of nociceptors

A

Alpha beta - low threshold mechanoreceptor (fastest)

Alpha delta - high threshold; cold, pressure, chemical

C - polymodal; temperature, chemical and pressure

66
Q

What is ‘nocifensive’ behaviour?

A

A sign of pain, affective response (e.g. jumping, withdrawal) and conditioned motor response (e.g. avoidance, escape, aggressiveness)

67
Q

How can the qualities and temporal features of pain be described?

A

First - stabbing/ pricking (acute)

Second - burning/ throbbing/ cramping/ aching; more affective to patients (chronic)

68
Q

Outline the neural pathway for nociception

A

First pain synapse -> dorsal horn -> central nervous system (including limbic system)

69
Q

Where do alpha-beta myelinated fibres synapse in the spinal cord?

A

PKC-gamma+ neurons

Lamina V

70
Q

Where do peptidergic C fibres synapse in the spinal cord?

A

Lamina I

Outer lamina II

71
Q

Where do alpha-delta myelinated fibres synapse in the spinal cord?

A

Outer lamina II

Lamina V

72
Q

Where do non-peptidergic C fibres synapse in the spinal cord?

A

Inner lamina II

73
Q

What is the general response of nociceptors to hot and cold stimuli?

A

As the temperature (hot temperatures increase) becomes more extreme, action potentials increase.
As soon as temperature reaches threshold, the number of action potentials are massively increased

74
Q

What are nociceptors?

A

Axons, cell bodies and central terminals of nociceptive dorsal root ganglion neurons

75
Q

Which nociceptors are the free endings generally associated with?

A

Unmyelinated C fibres

Thinly myelinated alpha-delta axons

76
Q

What is neurochemically understood for nociception?

A
  • Neurofillaments in alpha- beta fibres
  • IB4 (sugar molecule) positivity in alpha-delta
  • Neuropeptides (peptidergic C fibres) express CGRP, an important pain indicator
77
Q

How are noxious stimuli transduced with respect to action potentials?

A

As stimulus intensity increases, so does membrane potential until threshold is crossed and an action potential generated.

78
Q

What quality of nociception makes chronic pain a problem?

A

Nociception doesn’t really adapt but do fatigue
This means that for a patient suffering from chronic pain, it would come and go but remain consistent (with increased action potential firing before fatigue)

79
Q

How are different temperatures detected and integrated in nociceptive signalling?

A
  • Different transient receptor potential (TRP) channel combinations
  • Temperature detection by subset of DRG neurons
80
Q

What sensory neurons are responsible for detecting cold?

A

M8

A1

81
Q

What sensory neurons are responsible for detecting heat?

A

A1
V1 - 40-50 degrees
V2- 60 degrees

82
Q

What is the most famous gene associated with mechanoreception?

A

Piezo2

83
Q

What is spontaneous pain?

A

Pain that is not evoked by a stimulus

84
Q

What is the role of substance p with respect to pain?

A

Effector, released by DRG neurons and responsible for:

  • Vasodilation
  • Initiation of most cytokine expression
  • Amplifies/ excites most cellular processes
85
Q

What are chemical analogues for noxious temperature stimuli?

A

Capsacin - heat

Menthol - cold

86
Q

List examples of investigated candidates for the sensation of mechanical pain in mammals

A
  • OSM-9 (in C.elegans)
  • nompC
  • Nanchung
  • Painless (Drosophilia)
  • TRPA1 (mammalian hearing)
  • TRPP2 (mechanoreception in epithelial cells)
87
Q

How might peripheral sensitisation lead to pathological pain?

A
  • Augmented TRP channels -> sensitisation

- Modulation of sodium and potassium channels -> altered nociceptor excitability

88
Q

What are reactive oxygen species (ROS)?

A 
Any oxygen species that has the strong ability to accept electrons, reducing itself.
Effective but non-selective killers 
They include:
- Oxygen ions
- Free radicals
- Peroxides
89
Q

What is the difference between the outcomes for high and low oxidative stress?

A

High:
Increased expression of ‘atherogenic’ genes resulting in inflammation and vascular dysfunction

Low:
Increased expression of ‘atheroprotective’ genes - anti-inflammatory and protective for vasculature

90
Q

What is Nrf2?

A

NF-E2 related factor 2

Member of the cap-n-collar family; master regulator of cell responses against environmental stresses

91
Q

What is the role of Nrf2?

A

Induces expression of phase II detoxification enzymes and antioxidants

92
Q

List examples of ROS generated diseases

A
  • Diabetes
  • Pre-eclampsia
  • Atherosclerosis
  • Uraemia
93
Q

Describe KEAP 1 and its function

A

Kelch-like ECH-associated protein; subunit of ubiquitin ligase
Regulator of Nrf2

94
Q

How does KEAP 1 sense stress?

A

Its cysteine residues can detect different stress stimuli

95
Q

How does the Nrf2-KEAP 1 pathway change with redox?

A
  • Oxidation of KEAP 1 cysteine residues
  • Conformational change in Nrf2/ KEAP 1 complexes
  • Nrf2 forms heterodimers with May binds to antioxidant responsive element in promoter region of target genes
  • Expression results in production of proteins to help manage stress (e.g. HO-1)
96
Q

What are possible problems with excessive Nrf2?

A
  • Oncogenesis and cancer cell resistance
  • Overproduction of reduced glutathione and NADPH
  • Failure of corrupt differentiation in some cells
97
Q

What are the consequences of too little Nrf2?

A
  • Loss of cytoprotection
  • Diminished antioxidant capacity
  • Lowered beta-oxidation of fatty acids
  • Foetal pre-disposition to vascular disease
98
Q

What is oxidative stress?

A 
Protein oxidation that's poorly reversible by anti-oxidant systems; produced by the burden of ROS exceeding its antioxidant capacity
Can be caused by exposure to:
- UV radiation
- Environmental pollutants
- Cigarette smoke
99
Q

What are antioxidants?

A

Small molecules that possess redox- active properties, capable of scavenging ROS/ reactive nitrogen species.

100
Q

What is Nrf2 modulated by?

A

PKC

MAPKs

101
Q

What is the role of NQO1?

A

Phase II detoxifying enzyme stimulated by HNE

102
Q

How does Nrf2 play a role in pre-eclampsia and the offspring’s predisposition to vascular disease?

A
  • Less Nrf2 in foetal endothelial cells- less nuclear translocation and ARE binding in stress
  • Reduced HO-1 expression
  • Foetal cells more sensitive to HNE induced DNA fragmentation
  • Impaired antioxidant gene expression
103
Q

How is Nrf2 affected in gestational diabetes?

A
  • Diminished translocation to the nucleus -> less dimerisation with Maf
  • Less HO-1 induction via HNE in aortic smooth muscle
  • Compromised ARE genes from sustained activation via oxidative stress
104
Q

How is Nrf2 affected in ageing?

A
  • Decreased nuclear level

- Down-regulation of pathways like glutathione synthesis

105
Q

How is calcium usually found in cells?

A

Bound to calsequestrin in ER

Free/ bound to calmodulin in cytoplasm

106
Q

What is a general rule for the speed of different active transporters?

A

Primary - slow

Secondary - fast

107
Q

How can Nrf2 be degraded without KEAP1?

A

GSK-3beta phosphorylation via an adaptor protein (beta - TrCP)

108
Q

What is maf?

A

Antioxidant response element

109
Q

How does Nrf2 play a role in acute inflammation?

A

Regulates prostaglandin production through transcriptional regulation of: - Peroxiredoxins 1 and 6
- Liopocalin-type prostaglandin D synthase

110
Q

Why is Nrf2 a double edged sword?

A

Activation of some of the genes it binds to could increase resistance to chemotherapy in cancer (e.g. MRP1, p-gp, ABCG2)

111
Q

How does calcium signalling link to ROS?

A
  • Increased mitochondrial Ca2+ boosts ATP production
  • Increased ATP production means more oxygen is being reduced to water during oxidative phosphorylation
  • More reduction means more leakage of free electrons
  • More free electrons means more formation of superoxides
112
Q

Where are ROS generated by living cells?

A
  • NADPH oxidases
  • Xanthine oxidase
  • Lipoxygenase
  • Diaphorase
  • Mitochondrial electron transport chain (ETC)
  • Uncoupled endothelial nitric oxide synthase (eNOS)
113
Q

List examples of ROS that can cause protein modification

A
  • Peroxynitrite
  • Superoxide
  • Hydrogen peroxide
  • Hydroxyl radicals
114
Q

How does 4-hydroxynonenyl (4-HNE) modify proteins?

A

Michael Adduct formation; thought to lead to abnormal function

115
Q

How does peroxynitrite modify proteins?

A
  • S-nitrosylation of cysteine
  • Nitration of serine, threonine, tyrosine
    These all lead to modified function (unclear of whether this is bad)
116
Q

How do superoxide and hydrogen peroxide modify proteins?

A

Both cause reversible cysteine sulfenylation.

Problem: the more electrons you add, the more difficult it is to reverse the process.

117
Q

How is reversal of protein modification possible?

A

REDOX buffers (primarily glutathione) which are catalysed by reductases

118
Q

Give examples of ROS sensitive signalling in vascular smooth muscle

A
  • Tyrosine phosphatase inhibition (cysteine are oxidised)
  • Kinases (direct oxidation or oxidation of GPCRs)
  • GEFs (indirect activation)
  • Src activation
119
Q

How does ROS affect protein kinase C activity?

A

Depends on dose.
Low doses - stimulatory
High doses - inhibitory

120
Q

Why is the electron transport chain considered to be a hypoxia sensor?

A
  • Enhances leak of electrons, increasing superoxide production
  • Increased superoxide production -> increased ROS -> increased contraction
  • Block complexes, prevent superoxide generation, reduce ROS etc.
121
Q

What are the effects of rotenone and succinate?

A

Rotenone - blocks ROS production via complex I

Succinate - overrides effect of rotenone

122
Q

How does ROS cause contraction?

A

Increasing calcium influx into the cell -> activation of MLCK -> phosphorylation of MLC

Or MLCP inhibition:

  • Src (via PKC) activates CPI-17
  • Rho A (via ROCK) activates MYPT
123
Q

What are the effects of exogenous superoxide on the mesenteric artery?

A

Mixed:

  • Low doses - enhanced contraction
  • High doses - inhibited contraction
124
Q

What is endothelium-derived hyperpolarising factor (EDHF)?

A

Any factor that causes hyperpolarisation and therefore relaxation of underlying smooth muscle

125
Q

What is the difference between coupled and uncoupled eNOS?

A
  • Coupled-> vasodilation (product is nitric oxide)

- Uncoupled (via BH4 oxidation) -> vasoconstriction (product is hydrogen peroxide which is ROS and EDHF)

126
Q

Why is hydrogen peroxide more likely to be pro-relaxant?

A
  • It doesn’t activate the rho-kinase pathway or scavenge NO like peroxinitite
  • Likely just diffuses into vascular smooth muscle where it can activate K+ channels
127
Q

Why are ROS considered pro-proliferative?

A

They activate ERK and TRK which are both pro-proliferative

Physiology: References & Stats (4 decks)

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