1-Cell signaling-2-Ligand gated ion channels.pdf

cell signalling

2. Major Classes of Plasma Membrane Receptors
1. ION CHANNEL RECEPTORS
2. RECEPTORS THAT ARE KINASES OR THAT BIND TO AND ACTIVATE
KINASES
3. HEPTAHELICAL RECEPTORS

3. 2. RECEPTORS THAT ARE KINASES OR THAT BIND
TO AND ACTIVATE KINASES
•Signal transduction may involve two types of tyrosine
kinase:
1. receptors with inherent tyrosine kinase activity (RTKs), for
example insulin receptor;Receptors with tyrosine kinase
(RTK) activity
2. receptors, which recruit cytoplasmic tyrosine kinases, for
example growth hormone, leptinoperating through Janus
kinase (JAK).Receptors linked to tyrosine kinases,

5. Plasma membrane receptors are grouped
into 6categories:
•Gated ion channels.
•Receptors linked to membrane-bound G-protein -work
through second messengers;
•Receptors with tyrosine kinase (RTK) activity;
•Receptors linked to tyrosine kinases,
•Receptors withtyrosine phosphatase (RTPase) activity.
•Atrial natriuretic peptide receptor with guanylatecyclase
activity
This classification is based on the receptor's general structure and
means of signal transduction.

6. 2. Plasma Membrane Receptors and
Signal Transduction
•All plasma membrane receptors are proteins with
certain features in common: an extracellular domain
that binds the chemical messenger, one or more
membrane-spanning domainsthat are α-helices, and
an intracellular domainthat initiates signal
transduction.

7. •As the ligand binds to the extracellular domain of its
receptor, it causes a conformational changethat is
communicated to the intracellular domain through the α-
helix of the transmembranedomain.
•The activated intracellular domain initiates a characteristic
signal transduction pathway.
Signal transduction pathways run in one direction. From a
given point in a signal transduction pathway, events closer to
the receptor are termed “upstream”and events closer to
the response are termed “downstream.”

8. Signal transduction mediated by
chemically-gatedion channel
Signal transduction mediated by G-
protein coupled receptor
Signal transduction mediated by enzyme
coupled receptor
Cellular transmembrane signal
transduction

9. •In many pathways, the signal is transmitted by a
cascade of protein phosphorylations
•This phosphorylation (kinases) and dephosphorylation
(phosphatases) system acts as a molecular switch,
turning activities on and off
•Phosphatase -enzymes remove the phosphates

10. Signal molecule
Activated relay
molecule
Receptor
Inactive
protein kinase
1
Active
protein
kinase
1
Inactive
protein kinase
2 Active
protein
kinase
2
Inactive
protein kinase
3
Active
protein
kinase
3
ADP
Inactive
protein
Active
protein
Cellular
response
ATP
PP
P
i
ADP
ATP
PP
P
i
ADP
ATP
PP
P
i
P
P
P

11. Terms you need to know
•Agonistsarestructuralanalogs that bind to a receptor
and mimic the effects of its natural ligand.
•Antagonistsare analogs that bind the receptor without
triggering the normal effect and thereby block the effects
of agonists, including the biological ligand.
In some cases, the affinity of the synthetic agonist or
antagonist for the receptor is greater than that of the
natural agonist

12. The pathways of signal transduction for
plasma membrane receptors
The pathways of signal transduction for plasma
membrane receptors have two major types of effects
on the cell:
1.rapid and immediate effects on cellular ion levels or
activation/inhibition of enzymes
and/or
2. slower changes in the rate of gene expression for a
specific set of proteins. Often, a signal transduction
pathway will diverge to produce both kinds of effects.

14. 1. Ion Channel Receptors or
•Ligand-gated ion channels(LGICs) (ionotropic
receptor) or channel-linked receptor.
•They are opened or closed in response to the binding
of a chemical messenger (i.e., a ligand), such as a
neurotransmitter.
•Most small-molecule neurotransmitters and some
neuropeptides use ion channel receptors.
NOTE
The majority of ion channels fall into two broad categories: voltage-gated ion
channels (VGIC) and ligand-gated ion channels (LGIC).

15. Signal
molecule
(ligand)
Gate
closed
Ions
Ligand-gated
ion channel receptor
Plasma
membrane
Gate closed
Gate open
Cellular
response

16. •These proteins are typically composed of at least two
different domains: a transmembranedomain which
includes the ion pore, and an extracellular domain which
includes the ligand binding domain.

17. •LGIC is regulated by a ligandand is usually very selective to
one or more ions like Na
+
, K
+
, Ca
2+
, or Cl
-
.
•Such receptors located at synapsesconvert the chemical
signal of presynapticallyreleased neurotransmitter directly
and very quickly into a postsynapticelectrical signal.
•Many LGICs are additionally modulated by allostericligands,
by channel blockers, ions, or the membrane potential.
•Ligand-gated ion channelsare likely to be the major site at
which anaestheticagents and ethanolhave their effects,
halthoughunequivocal evidence of this is yet to be established

18. Example: nicotinic acetylcholine receptor
•The prototypic ligand-gated ion channel is the nicotinic
acetylcholine receptor.
•The job of a neurotransmitter is to change the membrane
potential of the postsynaptic cell and to do it quickly.
•The fastest and most direct mechanism isbinding of the
neurotransmitter to a ligand-gated ion channel in the
plasma membrane.
•The nicotinic acetylcholine receptor in the neuromuscular
junction is a classic example.

19. •This receptor is a channel for the monovalent cations sodium and
potassium, with five subunits that each contribute to the channel
(Fig.). The channel is closed in the resting state, opening only when
acetylcholine binds. Opening of the channel causes a rapid influx of
sodium down its electrochemical gradient, which depolarizes the
membrane.

20. This receptor is a ligand-
gated channel for small
cations (Na+, K+).
Acetylcholine binds with
positive cooperativity to
the two α subunits.
Each of the five
polypeptides traverses
the membrane four
times, and one of the
transmembrane helices
in each subunit
contributes to the “gate”
in the channel.
Structure of the nicotinic acetylcholine receptor in the
neuromuscular junction.

21. The channel complex consists of a pentamericcomplex.
Each a-subunit is glycosylated, and two other subunits contain covalently bound lipid. Each subunit has an
extracellular N-and C-terminus, four membrane-spanning domains (M 1, M2, M3, and M4), with a long
cytoplasmic loop between M3 and M4 that contains consensus sites for phosphorylation by protein kinases.
Five juxtaposed M2 domains (one from each protein subunit) form the ion-conducting pore.
The M2 domains of cation-selective receptor channels, such as the nicotinic-acetylcholine receptor,are enriched
in negatively charged amino acids at the intracellular "mouth" of the pore.
This region acts to attract cations like Na+ and repel anions like Cl-.
Binding of the neurotransmitter to extracellular sites on the complex induces very small, subtle rearrangements of the
M2 domains. This rearrangement removes the energetic barriers to ionic flow through the water-lined pore .
Phosphorylation of the a-subunit is also required for activity.
Closure of the channel occurs within a millisecond due to rapid hydrolysis of acetylcholine and dissociation of the
products, acetate and choline, from the protein .
Several deadly neurotoxins , including d-tubocurarine, the active ingredient of curare,
and several t0xins from snakes, including a -bungarotoxin, erabutoxin, and cobratoxin, inhibit the nicotinic-
acetylcholine receptor.
Succinylcholine , a muscle relaxant, opens the channel leading to depolarization of the membrane; succinylcholine is used
in surgical procedures because its activity is reversible due to the rapid hydrolysis of the compound after cessation of
administration .

22. •Structurally, succinylcholine is 2 Ach molecules linked together by methyl
groups. This binds and stimulates the Ach receptor on the postsynaptic
neuromuscular endplate, causing ion channels to open and sodium influx
to occur. Unlike Ach, succinylcholine produces continuous stimulation of
the nicotinic receptor, and the endplate membrane remains depolarized
with the channel open. The resulting skeletal muscle paralysis occurs
because the hydrolysis of succinylcholine is slow compared with Ach. This
sustained depolarization renders the postjunctionalmembrane unable to
respond to subsequent release of Ach because rapid fatigue of the muscle
occurs. In essence, the endplate and adjacent sarcolemma are refractory to
subsequent stimulation. Paralysis proceeds from the small, distal, rapidly
moving muscles to the proximal, slowly moving muscles.

23. Neuronal nAChRsare ligand-gated cation channels that are activated by the endogenous
neurotransmitter acetylcholine (ACh) and the exogenous tertiary alkaloid nicotine. They belong to
the superfamily of Cys-loop ligand-gated ion channels that include receptors for γ-amino butyric acid
(GABA, the GABAA, and GABAC receptor), glycine, and 5-hydroxytryptamine (5-HT3). These ligand-
gated ion channels have similar structural and functional features. All subunits in this family contain
a pair of disulfide-bonded cysteinesseparated by 13 residues (Cys-loop) in their extracellular amino
terminus.
Neuronal nAChRs, like all members of the cys-loop family of ligand-gated ion channels are formed by
the arrangement of five subunits to create a central pore. The structure of neuronal nAChRsis
homologous to muscle nAChRs. Each nAChRgene encodes a protein subunit consisting of a large
amino-terminal extracellular domain composed of β-strands, four transmembrane α-helices
segments (M1-M4), a variable intracellular loop between M3 and M4, and an extracellular carboxy-
terminus. The extracellular N-terminus contains the AChbinding domain that forms a hydrophobic
pocket located between adjacent subunits in an assembled receptor. The M2 segment of all five
subunits forms the conducting pore of the channel, and regions in the M2 intracellular loop
contribute to cation selectivity and channel conductivity.
Neuronal nAChRs

24. In vertebrates, 12 genes encoding 12 distinct neuronal nAChRsubunits have been
identified (Cholinergic Receptor Nicotinic Alpha: CHRNA2-10 and Cholinergic
Receptor Nicotinic Beta: CHRNB2-4 encoding α2-α10 and β2-β4 nAChRsubunits,
respectively) all of which can be found in humans and other mammals, except for
α8 which has only been identified in avian species.
Subunits are classified as either α-, by the presence of a Cys-Cyspair near the
start of TM (transmembrane)1,or non-α (β) when the Cyspair is missing.
Five subunits combine to form two classes of receptors: homomericreceptors
containing only α subunits (α7-α9) or heteromericreceptors that contain α and β
subunits (α2-α6 and β2-β4).
The most abundant subtypes in the brain are the low affinity α7 homomericand
high affinity α4β2.

25. Neuronal nAChRsubunits are classified as ‘α’ if they also contain the vicinal cysteines
required for agonist binding.
The muscle α subunit is designated α1, with α2-α10 being neuronal subunits.
However, α5 lacks certain amino acids from the binding site loops, so it is not a true α
subunit capable of contributing to the principal face of a binding site.
The α4 subunitnisalmost exclusively found in CNS neurons, whereas α9 and α10 are not
expressed in the CNS but are located in cochlear hair cells and some sensory neurons.
Subunits lacking the vicinal cysteinesare designated ‘β’, with β1 contributing to muscle
nAChRand β2-β4 expressed in various nonmusclecell types.

27. Acetylcholine (ACh) acts on two differenttypes of receptors:
Nicotinicand Muscarinic.
Nicotinic receptors(for which nicotine is an activator) are found atthe neuromuscular junction of skeletal
musclecells, as well as in the parasympathetic nervoussystem.
Muscarinic receptors (for whichmuscarine, a mushroom toxin, is an activator)are found at the
neuromuscular junction ofcardiac and smooth muscle cells, as well asin the sympathetic nervous system.
Curare (aparalyzing agent) is an inhibitor of nicotinicAChreceptors, whereas atropine is an inhibitor
of muscarinic AChreceptors.
Atropine maybe used under conditions in which acetylcholinesterasehas been inactivated by variousnerve
gases or chemicals such that atropinewill block the effects of the excess AChpresentat the synapse.

28. Myasthenia gravis is a disease of autoimmunity caused by the production of
an antibody directed against the acetylcholine (ACh) receptor in skeletal
muscle. In this disease, B and T lymphocytes cooperate in producing various
antibodies against the nicotinic AChreceptor.
The antibodies then bind to various locations in the receptor and cross-link
the receptors, forming a multireceptorantibody complex. The complex is
endocytosedand incorporated into lysosomes, where it is degraded
Myasthenia gravis
CLINICAL CORRELATION

30. Voltage-gated ion channels
Voltage-gated ion channelsare a class of transmembrane ion channels
that are activated by changes in electrical potential difference near the
channel; these types of ion channels are especially critical in neurons.
They generally are composed
of several subunits arranged
in such a way that there is a
central pore through which
ions can travel down their
electrochemical gradients.
The channels tend to be ion-
specific, although similarly
sized and charged ions may
sometimes travel through
them.

31. Voltage-gated ion channels
•Voltage-gated ion channelshave a crucial role in excitable
neuronal and muscle tissues, allowing a rapid and co-
ordinated depolarization in response to triggering voltage
change. Found along the axon and at the synapse, voltage-
gated ion channels directionally propagate electrical signals.

32. •Examples include:
•the sodium and potassium voltage-gated channels of nerve and
muscle.
•the voltage-gated calcium channls that play a role in neurotransmitter
release in presynaptic nerve endings.

34. •CLINICAL CORRELATION
•Lambert–Eaton MyasthenicSyndrome
Lambert–Eaton myasthenicsyndrome (LEMS) is an autoimmune
disease in which thebody raises antibodies against voltage gated
calcium channels (VGCC) located onpresynaptic nerve termini.
Upon depolarization of presynaptic neurons, calcium channelsat
presynaptic nerve termini open, permitting the influx of calcium
ions. This increase incalcium ion concentration initiates events of
the synapsincycle and leads to release ofneurotransmitters into
synaptic junctions. When autoantibodies against VGCC react with
neurons at neuromuscular junctions, calcium ions cannot enter
and the amount ofacetylcholine released into synaptic junctions is
diminished.