Ca signalling

Question 1

What are the components of the calcium toolkit?

Answer 1

On - cause calcium levels to be elevated through an influx of transporters or a release from stores
Off - depleting stores down to normal levels through buffers, pumps and transporters

Question 2

How does Ca carry out a diverse number of functions?

Answer 2

Small differences in spatial and temporal resolution

Question 3

What is the state of Ca at resting state?

Answer 3

• arounf 100nM concentration

- inhibitory function on gene transcription and neurite extension

Question 4

What are the sources of Ca?

Answer 4

• ER
• Mitochondria
• Acidic vesicles (lysosome - can cross talk with ER for larger Ca release)
• High levels of extracellular Ca allow for influx when required

Question 5

What is the main Ca receptor?

Answer 5

Ryanidine: 3 isoforms

Question 6

Which Ryanidine receptor is in the skeletal muscle?

Answer 6

Type 1: activated by mechanical coupling. Change in membrane potential activates Ca channel and dyhropyridine receptor. The latter change causes conformational difference allowing it to interact with RyR1 allowing Ca2+ to enter

Question 7

Which RyR is found in the heart?

Answer 7

Type 2 - Ca enters through v gated channels and then binds to this channel causing Ca mediated Ca release from stores

Question 8

How do IP3 receptors mediate Ca release?

Answer 8

Hormone activates G coupled receptor which activates PLC, which breaks PIP2 into DAG and IP3. IP3 then binds to RyR receptor to release Ca

Question 9

Describe store-operated Ca entry

Answer 9

ER has a STIM receptor, which has EF hands capable of sensing Ca levels. When low these can oligomerise to form a couple and coplex with ORAI1 at the plasma membrane channel. Through the ion channel TRIPC can allow Ca influx into the store

Question 10

What are the ‘on’ mechanisms of the Ca toolkit?

Answer 10

• SOCE mechanism
• Voltage operated Ca channels due to an electrochemical gradient
• Release from intracellular sores through IP3 aided by RyR
• Sensors of Ca changes e.g Troponin C alters conformation to allow actin and myosin to slide over eachother

Question 11

What are the ‘off’ mechanisms of the Ca toolkit?

Answer 11

• Buffers such as the mitochondira which contains calbindin proteins (parvalbumin)
• Pumps and exchanges
• Pumps on ER may pump Ca back into stores to be recycled
  (note: pumps against concentration require ATP)

Question 12

What are the different time domains Ca may act on?

Answer 12

Exocytosis - ms
Contraction - s
Transcription - min
Fertilization - days

Question 13

Give an example of an effect on Ca frequency and amplitude

Answer 13

• Hystidine binds to G-coupled receptors and activates IP3, at high concentration causes a high frequency and amplitude of Ca response
• In weaker response there is recruitment of less receptors on membrane and stores, therefore less release and less response

Question 14

Give an example of all the members of the toolkit for the cardiac myocite

Answer 14

stimulus - membrane depolarisation
Transducers - voltage gated Ca channels
Channels - bind to RyR/IP3R which if sufficient, binds to troponin C causing contraction
Pumps - Sarcoplasma Ca ATPase (SERCA) pumps Ca back into ER stores, calcuquestrin in ER binds up to 50 Ca to decrease gradient
Pumps - Ca/Na exchange and K/Na pump to avoid AP

Question 15

What happens in heart arrhythmia?

Answer 15

K/Na exchange is downregulated causing early afterhyperpolarisation which results in more Ca release, therefore less is available for the next contraction which is then weaker

Question 16

How can DAG also be involved in Ca signalling?

Answer 16

• PKC binds to RyR receptors to increase their activity

- DAG activates PKC

Question 17

How do mitochondria regulate Ca signalling?

Answer 17

• Act as Ca buffer
• Cell metabolism
• Cell survival
• Apoptosis

Question 18

How do mitochondria use Ca to drive apoptosis?

Answer 18

Alteration of Ca response drives trigger for opening of permeability transition pore which triggers release of cytochrome c and therefore apoptosis

Question 19

How do mitochondria act as a Ca buffer?

Answer 19

• located in close proximity with channels that elicit a rise in Ca on the ER and plasma membrane
• sense microdomains of high Ca
• Ca can easily cross outer mitochondrial membrane via VDAC, harder to cross inner membrane but can using specific transporter MICU