L2, Sensing with channels Flashcards
1
Q
Trp identification and naming:
A
- First identified in Drosophila
- Mutants found to have transient receptor potentials
- See WS1: Patch clamp technique
2
Q
Trp channel mechanistic overview: (Drosophila response to light)
A
- Rhodopsin (GPCR) senses light
- Metarhodopsin activated -> ..-> PIP2 -> IP3 + DAG
- Activation and opening of Trp channels
- Depolarisation -> Neuronal signalling
- i.e. Channels are closed in the dark, open in light -> AP
3
Q
Phototransduction in invertebrates:
A
- Light stimulates rhodopsin -> metarhodopsin
- cGMP -> 5’GMP
- Activated cNGCs (open in dark)
- Light leads to closing of channel -> hyperpolarisation
4
Q
Trp channel superfamily members:
A
- Many members
- Subdivided into TRPA, C, M, ML, P, V
5
Q
Distinguishing features of Trp channels:
A
- 6 TMS domains
- 1 pore domain -> all are cation selective (usually prefer Ca2+ over Na+)
- No voltage sensor
- Long C and N-termini for regulation of gating
- Highly promiscuous gating
- Key sites: Phosph. sites, ankyrin repeats, Calmodulin binding sites
6
Q
Regulation of Trp channels (broad categories):
A
- Receptor activation: GPCRs and RTKs that activate PLCs modulate TRP channels activity via PIP2, DAG or IP3-mediated Ca2+ release from internal stores
- Ligand activation: Either via exogenous small organic molecules (e.g. Capsaicin), inorganic ions and purine nucleotides
- Direct activation: Changes in ambient temperature or mechanical stimuli
7
Q
What TRP channels are involved in thermosensation?
A
- TRPV1, V2, V3, V4, M2 -> activated by heating
- TRPM8, A1 -> activated by cooling
- Different channels concerned with different ranges of temperature -> many together confer broad range
8
Q
Evidence for TRPV1 role (mice):
Thermosensation
A
- TRPV1 deficient mice lack a subset of neurons that respond to moderate heat
- Delayed response to painful heat
9
Q
Evidence for TRPM8 role (mice):
Thermo- and chemosensation
A
- Null mutant showed no preference for comfortable vs variable temperature chambers
- Wild type prefer comfortable chamber
- Neurons for TRPM8-/- mice show loss of cold sensitivity
- Lack of flinching to acetone
- Evidence for primary role in sensing
10
Q
Mechanism for thermosensing: TRPV1
A
- Channel opening shows a much steeper temperature dependence than channel closing
- As temperature increases, channel opening is favoured relative to channel closure
11
Q
Mechanism for thermosensing: TRPM8
A
- Temperature dependence for channel opening is much less steep than that for channel closing
12
Q
Mechanosensation and hearing:
A
- Fluid filled compartment of hair cells with stereocilia on top in ear
- Vibration transmitted into inner ear, moves fluid, stereocilia jostled from side to side
- Generates receptor potential in hair cells
- Rapid mechanism; can hear at frequencies from 200 to 20,00 Hz -> msec valuesM
13
Q
Mechanotransduction in hearing:
A
- Stereocilia consists of two structure joined by tip link; movement stretches out tip link -> depolarisation -> receptor potential transmitted along to brain
- Rapid repsonse; direct effect on ion channel
14
Q
Evidence for role of TRPA1 in mechanotransduction in hearing:
A
- TRPA1 highly expressed in hair bundles
- Maturation of hair cell mechanosensitvity coincides with TRPA1 mRNA expression
- Disruption of TRPA1 in zebra fish and mice inhibits mechanotransduction
- Structure of tail contains many ankyrin repeats -> anchoring to cytoskeleton to generate force upon mechanical stimulus
15
Q
TRP targets of sensory molecules (x4):
A
- Capsaicin, piperine: TRPV1
- Menthol, eucalyptol: TRPM8
