Sensing with Channels

Question 1

What are the 5 senses, and what ion channels are associated?

Answer 1

Photoreception.
Olfaction.
= cNGCs - downstream of specific GPCRs

Gustation.
Hearing.
Touch.
= TRP channels - are the receptor

Question 2

What sensations are associated with TRP channels?

Answer 2

Thermosensation (touch).

Mechanosensation (hearing).

Chemosensation (taste).

Question 3

What are memebers of the TRP channel superfamily?

Answer 3

TRPP
TRPML
TRPA
TRPC
TRPV
TRPM
(TRPN) - not in humans

Question 4

What are some of the distinguishing features of TRP channels?

Answer 4

6 TMS domains (S1-S6)

1 Pore (P) domain
= pore forming / selectivity filter
= all are cation selective with most having some preference for Ca2+ over Na+

No voltage-sensor (S4 TMS)

Long ‘C’ and ‘N’ termini
= regulation of gating
= highly promiscuous gating

Question 5

What are the 3 categories of regulating TRP channels?

Answer 5

(1) Receptor Activation

(2) Ligand Activation

(3) Direct Activation

Question 6

What does receptor activation involve?

Answer 6

GPCRs / receptor kinases that activate PLCs

= modulate TRP channel activity via PIP2, DAG or IP3 mediated Ca2+ release from internal stores
=e.g. light receptor in drosophila

Question 7

What does ligand activation involve?

Answer 7

Either via:

exogenous small organic molecules
(e.g. capsaicin)

inorganic ions
(e.g. Ca2+)

purine nucleotides
(e.g. ATP)

Question 8

What does direct activation involve?

Answer 8

Changes in ambient temperature and mechanical stimuli.

Question 9

Which TRP channels display temperature sensitive gating? (+how are they activated)

Answer 9

TRPV1, TRPV2, TRPV3, TRPV4
= activated by heating

TRPM8, TRPA1
= activated by cooling

= shown by using mutant knockout mice (have delayed responses to painful heat / environmental cold)

Question 10

What is the mechanism for thermosensing heat (TRPV1)?

Answer 10

TRPV1
= channel opening shows steeper temperature dependence than channel closing
= temperature increases = channel OPENING is favoured over channel closure

Question 11

What is the mechanism for thermosensing cold (TRPM8)?

Answer 11

TRPM8
= temperature dependence of channel opening is less steep than that for channel closing
= temperature increases = channel CLOSING is favoured over channel opening

Question 12

What is the molecular candidate for the transduction channel in hair cells using for mechanosensation and hearing? (+why)

Answer 12

= TRPA1

= highly expressed in hair bundles
= maturation of hair cell mechanosensitivity coincides with TRPA1 mRNA expression
= disruption of TRPA1 expression in zebra fish and mice inhibits MECHANOTRANSDUCTION

Question 13

How does motion of the hair bundle create a change in membrane voltage (mechanostransduction)?

Answer 13

Tip link connects sterocilia

Connected to the ion channel = gates it open

Mechanical stress, allows ion in

Depolarisation

= receptor potential

= very raid response = direct effect on ion channel

Question 14

What are some examples of targets for chemosensation?

Answer 14

Capsaicin = TRPV1 (hot)

Piperine = TRPV1

Menthol = TRPM8 (cooling)

Eucalyptol = TRPM8

Question 15

What was originally thought about Capsaicin vs now?

Answer 15

Capsaicin
= TRPV1 activation associated with eating chillies
= birds are indifferent to pain-producing effects

OG
=tyrosine-serine motid (Y511 and S512) in linker between TM2 and TM3 are essential for capsaicin binding
= though to bind between TM2 and TM3, inducing conformation change and channel opening

NOW
= Y511 and S512 = near binding site and mutations prevent binding of capsaicin
= capsaicin binding induces electrostatic interactions =pull on S4-S5 linker and stabilises the open channel state