Cellular Signaling II: Second Messengers

Question 1

What are secondary messengers

Answer 1

They are fast they cause signal transduction and signal ampllification

Question 2

What are some examples of second messengers

Answer 2

cAMP, cGMP, Ca, NO

Question 3

Explain the significance of alpha, beta and gamma subunits in activating the signal

Answer 3

All 3 of these subunits bind with their associated compounds downstream to start a signal cascade. Only the alpha subunit however is responsible for stopping the signal by phosphorylating the GTP

Question 4

What is the significance of AC (adenylate cyclase) in activating the signal? How does it do it and where does it reside?

Answer 4

It resides on the plasma membrane, it binds to the activated alpha subunit and intitiates singal cascade

Question 5

what is the function of AC

Answer 5

To make second messenger cAMP

Question 6

How does AC achieve its function? What is the biochemical basis of it.

Answer 6

It catalyzes the reaction of hydrolyzing the ATP and then it makes the phosphate group cyclic by making it react with the OH of the 3rd carbon

Question 7

What is the role of alpha subunit in AC reactions

Answer 7

Alpha subunit greatly enhances the speed of production of cAMP by AC

Question 8

What 2 alpha subunits can combine with AC. What are their respective effects

Answer 8

Alpha s and alpha i. They enhance the cAMP formation and inhibit the cAMP formation resepectively. Both are antagonistic towards each other

Question 9

What other compounds regulate the activity of AC

Answer 9

Some beta and gamma subunit also some forms of AC can be regulated by Calcium

Question 10

What does a cell do with cAMP

Answer 10

It interacts with EPAC and Protein Kinase A (PKA)

EPAC interacts with small G proteins, it is a guanine nucleotide exchange factor so it activates these G proteins and causes further downstream signaling by them

Question 11

What are the main subunits of PKA

Answer 11

They have regulatory subunits (that interact with cAMP) and catalytic subunits.

Catalytic subunits are released from the regulatory subunits when the cAMP binds to the regulatory subunits. Now the catalytic subunits will go on and phosphorylate stuff

Question 12

What are some of the functions of PKA

There are 4.

Answer 12

Main function is p’s (phosphorylation)

1. Heterologous desensitization, it desensitizes the GPCRs
2. It p’s the metabolic enzymes
3. P’s CREB which is a transcription facts. When it is ps by PKA it will go to the nucleus and regulate transcription, it will up regulate cAMP response elements or proteins, increasing the cell’s sensitivity to respond further to these signals
4. PKA also ps CFTR which is the defective protein in cystic fibrosis and it is also the substrate for cholera toxin. CFTR is then expressed on the cell surface membrane and then allows the excretion of Cl, which is then followed by NA and then with water

Question 13

How does cAMP affect CFTR

Answer 13

cAMP once made activate PKA which then ps CFTR protein which opens up and allows the chloride ions to leave the cell by facilitated diffusion. This creates a charge imbalance so NA follows the Cl and then water follows. This causes diarrhea which we associate with cholera

Question 14

What is the normal function of LH receptors

Answer 14

LH receptors are present in Leydig cells and they are responsible fro the production of testosterone which is under the control of cAMP. This process occurs under puberty and helps guide those cells

Question 15

What is the receptor activating mutation example of LH

Answer 15

In one mutation the receptor is such that it is activated even early on (and without the ligand I think, he didnt mention that) which leads to excessive production of cAMP leading to early spermatogenesis (this was observed in a 3 year old child).

Question 16

What is the mutation that disrupts the activation of LH receptor

Answer 16

LH receptor mutation causes inactivation of the receptor even in the presence of ligand, leading to a condition called pseudohermaphroditism. Puberty doesn’t take place in this condition

Question 17

What is the idea of signal amplification

Answer 17

The signals are in pm concentrations but the responses are milli molars as one receptor can make many cAMP and these cAMP can then activate many many PKAs.

Question 18

Other than cAMP and cGMP what other secondary messengers are there

Answer 18

Diacylglycerol (DAG), IP3 and Ca

Question 19

What kind of signalling receptors are associated with these secondary messengers

Answer 19

GPCRs

Question 20

How do these secondary messengers work

Answer 20

They work by activating the compound Phopholipase C (PLC) (it is a membrane bound protein). PLC cleaves PIP2 which is a membrane bound lipid.

When this cleavage takes place 2 different compounds are made, one is DAG which remains bound to the membrane and the other is IP3. IP3 is solouble so it goes to the cytoplasm for signalling

Question 21

What is needed by PLC to increase its cleavage activity

Answer 21

Alpha G subunit is also needed by PLC

Question 22

What are other factors that increase PLC activity

Answer 22

Some gamma and beta subunits, small G proteins and tyrosine kinases (TK)

Question 23

What happens downstream of IP3 and DAG

Answer 23

Protein Kinase C (PKC) is activated. Once DAG is formed PKC binds to it in the cell membrane and PKC is partially activated.

IP3 goes into the cytoplasm, inserts themselves into the membrane of the ER, releasing Ca which then floods the cytoplasm of the cell. It fully activates PKC

Question 24

What are the fates of Ca in a cell once it is released into the cytoplasm.

There are 4.

Answer 24

1. Full activation of PKC
2. Interacts with AC, some are activated by Ca and some are antagonized by Ca
3. it binds with phospholipase A2 (PLA2) which is involved in the development of inflammatory response
4. Ca also binds to a regulatory protein called Calmodulin which is a component of kianses and phosphatases so their activities are regulated

Question 25

What occurs during muscle contraction

Answer 25

Ca enters the cells through voltage gated channels, it binds to the ryanodine receptors in the SR, releasing more Ca in the cytoplasm leading to muscle contraction

Question 26

What does NOS do

Answer 26

It converts arginine to citruline and NO in the endothelial cells

Question 27

What is found in the smooth muscle cells that is involves in second messenger cascade

Answer 27

Guanylate cyclase (GC) which makes cGMP just like AC that makes cAMP

Question 28

What is the function of NO in smooth muscles

Answer 28

It goes to the smooth muscles from endothelial cells and stimulates cGMP production by interacting with GC. cGMP then leads to vasodilation

Question 29

How is the singaling for cAMP termianted

Answer 29

Phophodiesterase (PDE) cleaves the cAMP and this causes the the regulatory units on PKA to go back to the catalytic subunits and stop their kinase reactions

Question 30

How the cGMP signaling terminated

Answer 30

The same thing happens except that there is a specific PDE for cGMP, leading to stopping the smooth muscle relaxation

Question 31

How are the calcium signaling degraded

Answer 31

The IP3s are degraded and the DAG at the plasma membrane is degraded. Now we are only left with a pool of excess Ca in the cytosol.

This is removed by calcium binding molecules leading to complete inactivation of PKC

Question 32

How is calcium removed in muscle cells

Answer 32

After contraction there is a calcium ATPase in the membrane of the ER that utilizes ATP to pump Ca into the ER lumen. This ATPase is called SERCA.

Question 33

What is iNOS

Answer 33

He introduced to iNOS as NO radicals are made to kill pathogens

Question 34

How is NO regualted

Answer 34

iNOS mRNAs are very unstable and they are rapidly degraded so that we don’t produce too much of NO toxic free radicals

Question 35

What is cushing syndrome? What is the biochemical basis of the disease

Answer 35

It is obesity caused by excessive cortisol production. There is more corticotropin made which excited more melanocotrin 2 receptor which then makes more cAMP and this results in more PKAs which leads to excessive prodouction of cortisol

Question 36

What are the symptoms of cushing’s syndrome

Answer 36

Hypertension, impaired glucose tolerance, obesity, osteoporosis and depression

Question 37

What are the types of cushings syndrome

Answer 37

There are 2 types:
1. Corticotropin dependent form where there is increased production of corticotropin - aberrant production of corticotropin

2. Corticotropin independent form where the patients have simply high levels of cortisol

Question 38

What is the basis of corticotropin independent cushing’s syndrome

Answer 38

there is a mutation in the catalytic subunit of PKA - there is a mutation in the catalytic subunit that leads to the catalytic unit not being able to bind to the regulatory unit so there is excessive and autonomous cortisol production

Question 39

Drugs that are designed to help patients with angina

Answer 39

You give these patients calcium channel blocker which pretty much interferes with the capacity of the exxogenous calcium to enter the cell through the voltage gated channel. So this inhibits muscle contraction and the smooth muscles of the cardiac arteries remain relaxed allowing increased blood flow.

Question 40

How does nitroglycerin act on people having angina or MI

Answer 40

It is converted to NO and leads to smooth muscle relaxation

Question 41

What is the concept of cross talk

Answer 41

Leads to signaling molecules leading to modified outcome, different outcome and sometimes the same outcome

Question 42

Key terms

Answer 42

1. EPAC
2. Pseudohermaphroditism
3. PLC
4. PIP2
5. CREB
6. DAG
7. IP3
8. TK
9. PLA2
10. PKA and PKC
11. PDE
12. SERCA