Cellular Neuroscience (207) Ian Parker Lecture # 6 - Second messenger and Ca 2+ signaling.
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Transcript of Cellular Neuroscience (207) Ian Parker Lecture # 6 - Second messenger and Ca 2+ signaling.
Cellular Neuroscience (207)
Ian Parker
Lecture # 6 - Second messenger and Ca2+ signaling
Ionotropic and metabotropic receptors
Ionotropic (direct coupled) receptor. Receptor site and channel are part of the same molecular conmplex
Metabotropic (indirectly coupled) receptor. Ligand receptor and channel are distinct entities, expressed by different genes, and physically separated in the membrane. Functional link between them is via a diffusible ‘second messenger’ (or chain of several second messengers). (The extracellular ligand is the
‘first messenger’)
Discrimination between ionotropic and metabotropic responses using patch-clamp recording
Wall of patch-pipette
Ligand applied in bathing solution cannot get under rim of pipette
Ionotropic receptor would not activate, because ligand (neurotransmitter) cannot get to binding site on channel
trapped within patch.
Metabotropic receptor will activate channel within patch, because second messenger can diffuse within cytosol
Functional characteristics of ionotropic vs. metabotropic responses.
Metabotropic Ionotropic1. Amplification and diversity of responses A single receptor can generate many second messenger molecules, which can activate many channels; and activate more than one type of channel.
No amplification.Receptor site is part of same molecular complex as the channel, so can control only that channel. Also, a given receptor type will control only a single channel type.
2. Slow responsesChannel activation depends on complex pathway. Generation of messenger causes initial delay. Then response can persistlong after agonist is removed, owing to time taken for second messenger to be degraded
Fast responsesEverything is built into one molecular complex, so channel can open within microseconds of agonist application. Similarly, channel can close within milliseconds as agonist dissociates from receptor site.
3. Function depends on many moleculesComplex system, requires energy for generation and recycling of second messengers. Will not function in excised patches
Function depends on only a singlemoleculeNo energy needed. Channel continues to function in excised patch.
There are many different metabotropic receptors (more receptors than there are second messengers)
Neurotransmitter Metabotropic R Ionotropic R
Ionotropic receptors mediate fast through-put of information in the nervous system (e.g. when brake lights go on on truck ahead of you!). Metabotropic receptors are concerned with slower modulation of activity (e.g. wakefulness).
Second messengers serve many functions as well as directly controlling membrane ion channels
1. Direct action on ion channels. e.g. G-protein subunits on K+ channel, cGMP on channels in photoreceptors.
2. Modulation of channel activity. e.g. cAMP-dependent kinase acting via phosphoryation of channel protein.
3. Indirect control of channel activity via second messenger cascade. e.g. G-protein phospholipase IP3 Ca2+ opening of Ca2+-dependent K+ channel.
4. Modulation of enzyme activity
5. Regulation of gene transcription
Ca2+ signaling
Resting [Ca 2+ ] in cytosol maintained at ~ 50 nM by actions of pumps & exchangers. So, only a little Ca entering the cytosol will give big (a few M) increase in concentration.
Cytosolic Ca can……Activate membrane ion channelsRegulate enzyme activityModulate protein functionRegulate gene expression Kill cells! (necrotic & apoptotic death)
It has functions in virtually all cells of the body.Christened a ‘life and death’ messenger
Calcium is the ONLY link between electrical activity of a neuron (or any other cell) and the end response of the cell:
e.g. Ca2+ influx through voltage gated channels in presynaptic terminal evokes transmitter release
Ca2+ liberation from SR triggers muscle contraction (skeletal or cardiac muscle)
Ca2+ influx through ligand-gated channels involved in LTP
Unlike other ions (Na+, K+, Cl-) it is the CHEMICAL signal carried by Ca2+ that is important, not the electrical charge of the ion.
Sources of Ca2+; and Ca2+ permeable channelsExtracellular fluid (~ 2 mM)
Plasma membrane channels- Voltage-gated channels (N, P, Q, L-type)
Ligand-gated channels: e.g. several neurotransmitter-activated channels have appreciable Ca2+ permeability (nicotinic ACh, NMDA)
Store-operated channels: open in response to depletion of e.r calcium stores
Endoplasmic/sarcoplasmic reticulum (~ 1mM)
SR membrane (skeletal muscle)- Ryanodine receptors; opening coupled to voltage sensors in plasma membrane
SR membrane (cardiac muscle)- Ryanodine receptors; opening triggered by Ca2+ entering through plasma membrane voltage-gated channels
ER membrane- Ryanodine receptors; opening triggered by cytosolic Ca2+
IP3 receptors; opening requires both IP3 and cytosolic Ca2+
Only a few % of the Ca2+ ions in the cytosol are ‘free’
Most bind to stationary Ca buffers (proteins), which slow their diffusion
Ca2+ Ca2+
Ca2+ Ca2+ Ca2+ Ca2+
Diffusion coefficient for free Ca2+ in water is ~ 200 m2 s-1, in cytosol it is about 10 m2 s-1
Spatial and temporal aspects of Ca2+ signaling
Because Ca2+ diffusion in the cell is hindered by immobile buffers, transient Ca 2+ elevations can be highly localized, and specifically regulate only nearby targets – e.g. neurotransmitter release.
Or, Ca2+ ions can propagate as a wave throughout a cell: e.g. to communicate signals to nucleus
Sustained Ca2+ elevations kill cells (e.g. glutamate neurotoxicity). So Ca waves are generated periodically. Frequency of repetitive waves encodes stimulus strength.
‘Digital’ vs. ‘Analog’ encoding of information
Weak stimulus
Strong stimulus
Strength encoded by frequency or pattern of all-or none signals
Strength encoded in a continuously-graded manner by signal amplitude
Ca2+ Release Dynamics
Ca2+ Channel
Cell Membrane
Pump ER
IP3 Cytosol
IP3RPump
M 1.0]Ca[ Rest
[Ca]Local>10M
cytoplasm
Ca2+
IP3
+
IP3 receptor
Global Ca2+ signaling
Ca2+
waves in a
wholecell
[Ca
2+]c
yt
Global Ca2+ signaling
+ -
cytoplasm
Ca2+
IP3
+
IP3 receptor
+-
[Ca
2+]c
yt
Ca2+
waves in a
wholecell
Ca2+-induced Ca2+ release propagates Ca2+ waves
yx
Cytosol
z
Cytosol space
ER membrane
ER store
ChannelJ
ER
CytosolCaD
Membrane
10 M
Local & global Ca2+ signals
‘local’ Ca2+ signal
‘global’ Ca2+
signal
Model of local & global IP3 - evoked Ca2+ signals
IP3-dependent Ca2+ signals are ordered hierarchically
cellularmolecular
local
Ca2+
IP3
stochastic periodic