Question

Imagine if you could tinker with any component of the signal transduction pathway involved in vision in vertebrates. In this problem, you will predict the effect of a change in the activity of a single component of the signal transduction pathway in vision. The rod cell secretes the neurotransmitter glutamate onto the bipolar cell as a function of its membrane potential – the more depolarized, the more it secretes; the more hyperpolarized, the less it secretes. The bipolar cell in this case (ON bipolar) has glutamate receptors that cause closure of cation channels, hyperpolarizing their membrane. They synapse with ON ganglion cells and secrete acetylcholine into the synapse at a rate that increases with increased depolarization of the bipolar cell membrane. The ON ganglions have acetylcholine receptors that are ligand-gated ion channels. The ganglion cell responds by sending a train of action potentials to the brain; the frequency of these action potentials is a function of the amount of acetylcholine that they sense. In your experiment, you will expose the rod cells to a controlled light pulse (same length and intensity every time) and measure the output of the ON ganglion cells to which they are connected. Then you will repeat this in each perturbed system. In each case, predict whether the frequency of action potentials sent out by the ON ganglion in response to light will be HIGHER or LOWER than the unperturbed system.

• You express a version of rhodopsin that stabilizes 11-cis retinal relative to all-trans retinal                              [ Select ]                       ["HIGHER", "LOWER"]      
• You mutate the transducin (G_t) beta subunit such that it has much higher affinity for the transducin (G_t) alpha subunit bound to GTP (G_tα-GTP)                              [ Select ]                       ["HIGHER", "LOWER"]      
• You mutate the inhibitory subunit of cGMP phosphodiesterase (PDE) such that it has lower affinity for the catalytic subunit of PDE.                              [ Select ]                       ["HIGHER", "LOWER"]      
• You add a drug that mimics the effect of cGMP: it binds to the ligand-gated Na⁺/Ca²⁺ channel and opens it. However, this drug is not affected by PDE.                              [ Select ]                       ["HIGHER", "LOWER"]      
• You use CRISPR/Cas9 to delete the gene for rhodopsin kinase.

Answer 1

1.

LOWER

Explanation: Rod cell contains the rhodopsin in 11-cis conformation. Conversion of cis to trans is important to initiate the signaling. If you stabilize the 11-cis conformation, then signaling will not turn on and hence LOWER responds to light.

2.

LOWER

Explanation: In the deactivated form alpha subunit of the transducin has GDP and binds with beta and gamma subunit. When transducin gets activated, it dissociates from beta and gamma subunit and exchanges GDP with GTP. This activated form increases signaling. If GTP bound alpha subunit will bind to the beta form of mutant transducin, then alpha subunit exchanged its bound GTP with GDP and LOWER the signal.

3.

HIGHER

Explanation: activated alpha transducin subunit activates PDE. Enzyme PDE converts cGMP to 5’-GMP and propagates the signal. If you mutate the inhibitory subunit of PDE then enzyme PDE will remain active and more 5’-GMP will form and this leads to HIGHER signal.

4.

HIGHER

Explanation: Opening of cation channels leads to depolarization and hence more glutamate secretion and more acetylcholine secretion and HIGHER the signal.

5. Rhodopsin kinase phosphorylates the serine/threonine amino acids of the rhodopsin molecule. This phosphorylated rhodopsin binds with the arrestin protein and turns off the signaling pathway. If you delete the rhodopsin kinase gene, then phosphorylation will not take place and a signal will remain ON.