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Cellular Neuroscience > Ion channels and Transporters > Flashcards

Ion channels and Transporters Flashcards

1
Q

2 major subclasses of ion channels

A
  • Voltage-gated
  • Ligand Gated
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2
Q

Currents carried by Na+ are

A

Inward at potentials more negative than ENa and reverse their polarity above ENa

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3
Q

Properties of single Na+ channels

The amplitude of current depends

A

on Na+ concentration

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4
Q

Properties of single Na+ channels

Time course of opening, closing and inactivation matches

A

macroscopic current

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5
Q

macroscopic current

A

stochastic events averaged many times.

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6
Q

Properties of single Na+ channe

Opening and closing of channels are

A

voltage-dependent

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7
Q

Properties of single Na+ channels

Tetrodotoxin blocks

A

both microscopic and macroscopic Na+ currents.

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8
Q

properties of single channel K+

single channel K+ currents reflect

A

macroscopic currents

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9
Q

single channel K+ currents are…(Inward or Outward)?

A

Outward currents

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10
Q

during
brief depolarizations, single channel K+ channels…

A

Do not inactivate

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11
Q

single channel K+ channels are

A

voltage
-dependent

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12
Q

single channel K+ channels

Depolarization (increases or decreases) probability of opening

A

increase

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13
Q

single channel K+ channels

Hyperpolarization (increases or decreases) probability of closing

A

Increases

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14
Q

single channel K+ channels

single channel K+ channels are blocked by drugs that….

A

affect the macroscopic
current

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15
Q

Voltage-gated Ion Channels

Voltage-gated Ion Channels show…

A

ion selectivity

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16
Q

Voltage-gated Ion Channels

voltage-sensor

A

depolarization increases open
probability, while hyperpolarization
closes them

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17
Q

Voltage-gated Ion Channels

Which channel has a mechanism for inactivation?

`

A

Na+

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18
Q

Levels of protein structure

primary structure

A

The properties of a protein are determined by its
amino acid sequence

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19
Q

Levels of protein structure

secondary structure

A

Active proteins require the folding of polypeptide
chains into precise 3
-dimensional conformations (linked via hydrogen bonds).
Depending on the nature and arrangement of the
amino acids present

The 3D
structure is the thermodynamically most stable
configuration.

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20
Q

Levels of protein structure

alpha helices

A

Secondary structure in the shape of a coil

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21
Q

Levels of protein structure

beta sheets

A

Secondary structure with a flat, folded shape

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22
Q

Levels of protein structure

tertiary structure

A

Further folding and reorganization within the
molecule results in higher order

Occurs when ertain attractions are present between alpha helicies and beta/pleated sheets

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23
Q

Levels of protein structure

Quaternary structure

A

A question consisting of more han one amino acid chain

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24
Q

X-ray crystallography

A

A beam of X-rays strikes a crystal/protein and scatters into many different directions

From the angles and intensities of these scattered beams, one can produce a three-dimensional picture of the density of electrons within the crystal/protein

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25
Q

X-ray crystallography determines…

A

the arrangement of atoms within a crystal.

the mean positions of the
atoms in the crystal can be determined, as well as theirchemical bonds.

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26
Q

X-ray crystallography is used to…

A

determine how a drug interacts with its protein target and how this interaction can be improved

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27
Q

X-ray crystallography of membrane proteins is challenging because

A

it requires detergents to solubilize them in
isolation and such detergents often interfere with crystallization.

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28
Q

Cryogenic-electron microscopy

In Cryogenic
-electron microscopy

A

a biological sample is flash frozen
(vitrified), sliced, and then imaged using an
electron microscope

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29
Q

Cryogenic-electron microscopy has allowed for

A

the
determination of biomolecular
structures at near -atomic
resolution (~1.25
-ångström)

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30
Q

Molecular Structure of Ion Channels

Hetero-oligomers

A

constructed from
distinct subunits

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31
Q

Molecular Structure of Ion Channels

Homo-oligomers

A

constructed from
a single type of subunits

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32
Q

Molecular Structure of Ion Channels

single polypeptide chain

A

organized into repeating
motifs, each motif
functioning like a subunit

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33
Q

Molecular Structure of Ion Channels

auxiliary subunits (β or γ)

A

modulate the gating characteristics of
the central core

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34
Q

Molecular Structure of (typical) voltage-gated Ion Channels

The pore-forming subunits
(α-subunit) of the voltage-gated
Na+, Ca2+, and K+ channels are

A

composed of a common repeated domain contains
* 6 alpha-helical regions (S1-S6) and a
* P region (“Pore loop”) that goes in and out of the membrane.
* P region confers ion selectivity
* S4 is positively charged
and represents the voltage sensor

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35
Q

________ K+ channel subunits form a channel

A

Four

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36
Q

The Na+ channel

The Na+ channel consists of…

A

A pore-forming α subunit
associated with auxiliary β subunits

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37
Q

The Na+ channel

The α subunits are organized in

A

four homologous domains (I–IV), which each contain six transmembrane alpha helices (S1–S6)
and an additional pore loop located between the S5 and S6 segments.

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38
Q

The Na+ channel

The S5 and S6 segments

A

line the inner cavity and form the activation gate (confer ion selectivity)

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39
Q

The Na+ channel

S4 segments

A

Positively charged amino acid residues in the S4 segments serve as gating charges that move in response to depolarization.

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40
Q

The Na+ channel

The inactivation gate…

A

The short intracellular loop connecting homologous domains III and IV serves as the inactivation gate, folding into the channel
structure and blocking the pore from the inside during sustained depolarization.

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41
Q

The Na+ channel

β subunits

A

modulate the kinetics and voltage-dependence of channel gating, and they are involved in channel localization and
interaction with cell adhesion molecules, extracellular matrix and intracellular cytoskeleton.

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42
Q

Voltage sensor of the Na+ channel

A

S4 (red) = voltage sensor (positively charged amino acids)

depolarization causes conformational change in channel

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43
Q

Cycle of Na+ channel states

A

Rapid opening (activation) followed
by slower closing (inactivation)

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44
Q

Recovery of inactivation

of Na+ channels

Two-pulse voltage clamp protocols

A

test the kinetics of channel gating

During the intgerpulse interval, come channels recover from inactivation

2nd pulse determines what fraction have recovered in that time

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45
Q

The relative contribution of the persistent Na+
current (INaP) becomes more obvious at

A

depolarized potentials where the fast Na+ current is
inactivated

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46
Q

The late openings in single-channel recordings suggest that INaP is

A

generated by different kinetic modes of the same sodium channel,
with the same channel occasionally entering an open state that
lacks fast inactivation.

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47
Q

INaP is activated in

A

the subthreshold
voltage range

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48
Q

INaP serves to amplify
the response to

A

synaptic input and it
enhances repetitive firing capabilities.

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49
Q

INaP consists of

A

Only ~0.5–5% of the maximum
transient sodium current, but the
resulting current (5–200 pA) is
functionally very significant at
subthreshold voltages.

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50
Q

Most abundant Na+ channels - α subunits in adult CNS

A

Nav1.1, Nav1.2, and Nav1.6

Similar properties (subtle differences in voltage-dependence and activation/inactivation).
Their functions are non-overlapping. Knock-out of either is lethal.

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51
Q

Four major classes of K+ channels,
grouped by TransMembrane domains:

A

Tandem pore domain potassium channels (4TM)

Voltage-gated potassium channels (6TM)

Calcium-activated potassium channels (6 or 7TM)

[Inwardly rectifying potassium channels (2TM)]

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52
Q

Tandem pore domain potassium channels (4TM)

A

constitutively open or
possess high basal activation, such as the “resting potassium channels” or
“leak channels” that set the negative membrane potential of neurons.

53
Q

Voltage-gated potassium channels (6TM) -

A

voltage-gated ion
channels that open or close in response to changes in
the transmembrane voltage

Three subtypes
1. delayed rectifyer
2. A type
3. KCa2+ (BK, SK, IK)

54
Q

Calcium-activated potassium channels (6 or 7TM)

A

open in response to the
presence of calcium ions or other signaling molecules

55
Q

Voltage-gated K+ channel

voltage gate S4; Depolarization…

A

pulls on the S4-S5 linker to open the pore

56
Q

Kv

A

Voltage-gated K+ channels

57
Q

Delayed rectifiers Inactivate**

A

slowly or not at all

58
Q

Delayed rectifiers are further divided by…

A

by their activation kinetics
* Fast
* Slow

And their voltage-sensitivity
* High
* Low

59
Q

A-type inactivates

A

Inactivate rapidly

AKA Transient currents

60
Q

The classic A-type Kv
channels (Kv
1.4 and Kv
4) are

A

low-voltage activated

61
Q

Mammals gave 17 voltage-gated
K+ channel (Kv) genes, within
4 subfamilies related to

A

Shaker (Kv1.1-Kv1.8),
* Shab (Kv2.1 and Kv2.2),
* Shaw (Kv3.1-Kv3.4), and
* Shal (Kv4.1-Kv4.3)

62
Q

the first cloned K+ channel from a
mutant Drosophila fruit fly (1987)

A

Shaker

63
Q

Cloning of Shaker gene (1987) allows

A

first identification
of amino acid sequence of a channel gate
(→ inactivation gate).

Demonstration of “ball-and-chain” mechanism that had
been first hypothesized for Na+ channels
* However, a different part of the protein is involved and
there are 4 inactivation gates instead of 1, because K+
cannels are oligomers (4 subunits)

Inactivation is modulated by N-terminus

64
Q

Functions of Delayed Rectifiers

A
  • Resting potential
  • AP Threshold
  • AP shape (repolarization and hyperpolarization)
  • Membrane excitability
65
Q

Functions of A-type channels

A
  • Spike frequency coding
  • Dendritic Processing
66
Q

HVA Kv channels shape

A

action potentials and contribute to firing-related changes in
excitability

Located on the soma, nodes between myelin, presynaptic

67
Q

LVA Kv channels

A

keep excitability in check (Kv1, Kv7)
*
Located on Axon nodes

Kv7 provide the “M-current” (muscarinic), which limits firing rate

68
Q

A-type channels are expressed in

A

dendrites and are active at low membrane potentials

69
Q

In dendrites, A-type currents limit depolarization to

A

active synapses, enabling synapse-specific
plasticity to occur.

70
Q

A-type currents help to

A

slow down depolarization after an AP (but not too much since they
inactivate), enabling frequency adaptation of AP firing (→ “rate coding”)

71
Q

A
-type currents

A

regulate dendritic excitability

72
Q

IA increases in

A

distal dendrites and reduces EPSPs
* Counteracts (local) EPSP amplification by Ca++
currents, INaP, as well as bAPs

73
Q

A type channels are blocked b y

A

4-AP

74
Q

The intensity of signals is coded through

A

the frequency of APs

75
Q

rate coding

A

Cells respond to an increase in inputs with an increase in AP firing

76
Q

Neurons receive depolarizing inputs
at the

A

dendrites, the soma, and at the
axon hillock

77
Q

Axons transmit APs that

A

result from
summed inputs
* Spike rate is a function of depolarization
* Higher input → higher firing rate

78
Q

A-type currents

Kv4.x and Kv1.4 subtypes open

A

briefly only at relatively low membrane potentials
(i.e. during AHP between spikes)

79
Q

A-type currents

A-type currents provides

A

a hyperpolarizing current that pulls membrane from AP threshold
* Lengthens time between spikes,
* But also allows more Na+ channels to recover from inactivation

80
Q

With higher input
intensities,

A

membrane
potential does not
hyperpolarize as much
* Fewer KA channels recover
from inactivation
→ neuron can fire faster

81
Q

High-voltage activated delayed
rectifiers open

A

on depolarization
* In the axon (e.g. Kv3) HVAs
contribute to shaping of the AP
* In the soma (e.g. Kv2) HVAs
regulate excitability

82
Q

Low-voltage activated delayed
rectifiers are open

A

below AP
threshold (and above)
* In the axon (e.g. Kv1, Kv7) they
regulate excitability, AP shape, and
firing rate

83
Q

β
-subunits can alter

A

inactivation * pharmacology * regulation (e.g. ATP sensitivity)

84
Q

Calcium-activated potassium channels (6 or 7TM) has three subtypes:

A

BK – Big potassium channel (KCa1.1)
SK – Small potassium channel
IK – Intermediate potassium channel

85
Q

SK current activates

A

slower than BK

86
Q

SK

A

helps shape the afterhyperpolarization.

contributes little to the fast
repolarization of the action potential,

87
Q

The duration of SK conductance reflects

A

the decay of intracellular free calcium
(>100 ms).

88
Q

BK channels deactivate

A

far more quickly, since both depolarization
and high local intracellular calcium are
required for activation

89
Q

BK channels (KCa1.1) –
large conductance Ca2+
-activated K+channels are sensitive to

A

TEA and charybdotoxin (CTX,
scorpion venom)

90
Q

At the soma, BK

A

mediate rapid spike
repolarization and fast
afterhyperpolarization.

91
Q

In dendrites, BK

A

regulate the duration of
dendritic calcium spikes and burst firing.

92
Q

SK channels (KCa2+) - Ca2+-activated K+channels have a

A

Smaller conductance (10-20 pS) than BK channels, but more sensitive to Ca++

93
Q

SK channels are nly (weakly or strongly) voltage-dependent

A

weakly

94
Q

SK channels are sensitive to

A

apamin (bee venom)

95
Q

Regulators of K+ conductance (RCK) domains encode “calcium bowls”
(Ca++ binding sites)

A

Only on BK channels

96
Q

SK channels are modulated by

A

modulated by calmodulin
* Also activated by Ca++ spikes, or by Ca++ influx through NMDARs

97
Q

SK channels contribute

A

to spike frequency adaptation.

98
Q

All K+ channels discovered so far possess a core of alpha
subunits, each comprising

A

g either one or two copies of a
highly conserved pore loop domain (P-domain).

99
Q

Ions must

A

shed water before they can pass through channels

100
Q

Na+
is a smaller molecule than K+,
but its effective size is

A

is larger than
K+
, because it attracts a larger
sphere of water

101
Q

The selectivity filter in a NA+ channel

A

is just large enough to
accommodate one Na+
ion contacting one water
molecule.
This involves transient binding and stabilizing of the
positive Na+
ion with a negatively charged amino acid
in the wall of the pore.Cations that are larger in diameter (e.g. K+
) cannot pass
through.

102
Q

on-selectivity of channels - K+ channel

A

As K+ passes through the pore, interactions between K+
ions and water molecules are
prevented and K+
is stabilized by interacting with specific components of 8 amino acids that in
all K+ channels include the sequence TxxTxGxG (the K
+
selectivity sequence)

103
Q

Ion-selectivity of channels - K

+ channel

The walls of a K+ channel are

A

too far apart to stabilize a dehydrated Na+ ion

104
Q

Ion-selectivity of channels - K+ channel

The Negatively charged pore helix

A

Strips water from K+
ions
so they fit through the filter

105
Q

Ca++ channels and α-subunits

α-subunits consist of

A

Single polypeptide chain. 4 repeats of a domain that contains
6 alpha-helical regions (S1-S6) and a P region,
plus 3 ancillary subunits α2, β, and γ.

106
Q

Ca++ channels and α-subunits

Three genes for α subunits (Cav1-Cav3)

A

High-threshold channels (HVA)
* Cav1.x (L-type channels),
* Cav2.1 (P/Q-type channels), and
* Cav2.2 (N-type channels) show little inactivation.
Functionally similar, but distinguished by pharmacology (sensitivity to omega Ѡ-conotoxins)

Low-threshold channels (LVA)
Cav3.x (T-type channels)
inactivate rapidly

107
Q

The L-, N-, and P-type Ca++ channels
are

A

high
-threshold channels (HVA)

108
Q

L-type Ca++ channel

A

large, long
-lasting

109
Q

N-type Ca++ channel

A

Neuronal

110
Q

P-type Ca++ channel

A

Purkinje cell

111
Q

The probability of the L type channels being in the open state

A

inreases with depol so that they overlap and lead to a suden increase in corresponding macroscopic current.

112
Q

T-type Ca++ channels
are

A

low-threshold channels (LVA)

Tiny conductance, transient activation

113
Q

T-type Ca++ channels become

A

de-inactivated during
cell membrane hyperpolarization, and
then open again to depolarization.

114
Q

T-type channels are important for

A

rhythmic firing patterns in cardiac
muscle cells and thalamic neurons.

115
Q

T-type channels are activated at

A

very
negative potentials (
→LVA) where
there is a large driving force for
calcium going into the cell.

116
Q

T-type channels fast voltage-dependent inactivation

A

also allows for more
frequent depolarization.

117
Q

Optogenetics

A

Light-gated channels

118
Q

Two types of Light-gated channels

A
  1. Channelrhodopsin (ChR)
  2. Halorhodopsin (HR)
119
Q

Channelrhodopsin (ChR)

A

Are not ion selective
Activated by blue light
Excitability (ChR2)

120
Q

Halorhodopsin (HR)

A

Allow only choride
Activated by yellow light
inhibition (NpHR, or Arch)

121
Q

Active Transporters

A

create and maintain Ion Gradients

122
Q

Active Transporters require

A

energy (ATP, or electrochemical gradient of other ions)

123
Q

Kinetics of Active Transporters

A

slow (e.g. several ms for 3 Na+
ions vs. >1000 Na+
ions per ms
through Na+ channel during AP.

124
Q

Two subtypes of Active Transporters

A
  • P-type ATPases
  • Ion exchangers(use electrochemical gradient):
125
Q

Two types of P-type ATPases

A
  1. Na+/K+ ATPase pump
  2. Ca++ pump
126
Q

Three types of Ion Exchangers

A
  1. Na+/Ca2+ exchanger
  2. Na+/K+/Cl- co-transporter
  3. K+/Cl- co-transporter
127
Q

The sodium-potassium pump (Na+/K+ATPase)

3 Na+
ions are carried out for

A

every 2 K+
ions brought in
→ net loss of 1 positive ion
inside (pump is electrogenic)
→ hyperpolarizes cell

128
Q

The sodium-potassium pump (Na+/K+ ATPase) accounts for

A

20-40% of brain’s energy consumption
Uses about 3 million ATPs per second (in rod photoreceptors, which have high resting Na+
permeability the number is 60 million ATPs per second)

129
Q

Antiporters

A

Typically coupled to Na+ movement

Ca++ and H+(pH regulation) are
exported from cells by antiporters which couple their export to the energetically favorable import of Na+

Cellular Neuroscience (10 decks)

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