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Cells to Organisms - Lectures > Lecture 10. Calcium Dependent Exocytosis > Flashcards

Lecture 10. Calcium Dependent Exocytosis Flashcards

1
Q

What is on the extracellular side of the membrane ?

A

A local positive charge on the outside of the membrane

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2
Q

What is on the cytoplasmic side of the membrane ?

A

A local negative charge on the inside of the membrane

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3
Q

What does the charge separation at the membrane result in ?

A

Resting membrane potential

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4
Q

How does the potential arise ?

A

If the positive charges are allowed to, they will move back down across the gradient into the interior of the cell

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5
Q

What allows the potential to be held within the resting membrane potential ?

A

Holding the positive charges outside the cell against the concentration gradient

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6
Q

What are two things that contribute to the potential ?

A
  1. Chemical gradient

2. Electrical gradient

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7
Q

What are the proteins embedded in the membrane that influence the potential ?

A
  1. Potassium leak channels

2. Sodium potassium ATPase

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8
Q

What happens when potassium leak channels are accessible ?

A

Ions move out of the cell down their concentration gradient to deplete the extent of the potassium gradient between the inside and outside of the cell

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9
Q

What does sodium potassium ATPase do ?

A

It constantly hydrolyses ATP. It pumps out the three sodium ions and co-transports inwards two potassium ions. This transaction increases the positive charge outside the cell increasing the charge separation

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10
Q

What is the concentration of sodium outside the cell ?

A

120mM

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11
Q

What is the concentration of sodium inside the cell ?

A

16mM

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12
Q

What is the concentration of potassium outside the cell ?

A

63.8mM

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13
Q

What is the concentration of potassium outside the cell ?

A

3mM

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14
Q

What is the function of the pump ?

A

Maintains the concentration gradients and the charge separations and in that is stored the membrane potential

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15
Q

What allows the neuron to do work ?

A

The membrane potential

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16
Q

What causes the neuron to fire and release the neurotransmitter ?

A

Depolarisation

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17
Q

What is a huge amount of the neuron’s energy budget spent on ?

A

Pumping sodium out and potassium in

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18
Q

When do neurons typically depolarise ?

A

When gated ion channels open when a signal binds to these channels

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19
Q

What causes the channels to open and ions to flow in ?

A

The signal binds to the receptor on the gated ion channel which causes it to change its shape

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20
Q

What happens when a ligand binds to the channel ?

A

The ion channel is gated by the ligand and allows ion in causing sodium to rush in down the concentration and charge gradient

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21
Q

What does the resting membrane potential tend to be around ?

A

-65-70 mV

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22
Q

What happens when sodium ions rush into the negative cell ?

A

They depolarise the cell making it more positive. The resting membrane potential moves upwards towards zero because of the influx of positively charged sodium cations

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23
Q

What is a small depolarisation known as ?

A

A failed initiation that is insufficient to trigger the firing of an action potential

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24
Q

What are the two types of sodium gated channels ?

A
  1. Ligand gated sodium channels

2. Voltage gated sodium channels

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25
Q

What causes the first channels to open ?

A

The ligand binding and allows sodium to flow into the channel, changing the voltage of the membrane but not enough to cross the threshold

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26
Q

What causes the voltage gated ion channels to open ?

A

The change in voltage caused by the opening of ligand gated sodium channels, further depolarising the cell and allowing the potential to reach the threshold

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27
Q

What is end plate potential ?

A

When a neuron synapses onto a muscle cell

28
Q

What is action potential propagation ?

A

Sodium moves into the cell and depolarises an area of the cell. It also opens further voltage gated channels nearby. This depolarisation process is repeated all the way down the axon in one direction only

29
Q

What stops the depolarisation going backwards ?

A

Voltage gated potassium channels opening behind the sodium voltage gated ion channel

30
Q

What is the concentration of potassium inside the cell verses outside the cell at the presynaptic terminal ?

A

400mM vs 20mM

31
Q

What is the concentration of sodium outside the cell verses inside the cell ?

A

400mM vs 50mM

32
Q

What happens when you block sodium channels with tetrodoxin ?

A
  1. Prevents sodium channels from opening

2. There is no presynaptic potential and no resulting post synaptic potential

33
Q

What does TEA do ?

A

Shows the post synaptic potential does not repolarise

34
Q

What happens if the two toxins are combined ?

A

There is less and less of the post synaptic potential to start with and doesn’t resolve at all

35
Q

What happens if you block potassium channels ?

A

Neurotransmission is not prevented but the neurons fail to repolarise after the action potential has been triggered

36
Q

What is the electrical trigger for depolarisation ?

A

Sodium

37
Q

What is the chemical signal for neurotransmitter release ?

A

Calcium

38
Q

What does sodium do ?

A

Changes the voltage and depolarisation opens the voltage gated calcium channels

39
Q

What does calcium do ?

A

Triggers the changes in biochemistry of the vesicle allowing vesicles to fuse with them membrane and release contents (exocytosis)

40
Q

What happens when sodium enters the channels in the region of the presynaptic terminal ?

A

It depolarises the entire terminal

41
Q

Why do voltage gated calcium channels open ?

A

Because of the change in voltage caused by the depolarisation of the presynaptic terminal

42
Q

What does calcium do ?

A

Enters the cell and facilitates neurotransmission

43
Q

What is the synaptic vesicle cycle ?

A
  1. Vesicles have to be filled and cluster in the active zone.
  2. A small number have to dock at the active zone where they are primed for calcium dependent exocytosis.
  3. Calcium arrives and they must fuse with the membrane before recycling.
44
Q

What do vesicles not do ?

A

Cross the synaptic cleft

45
Q

What is an active zone ?

A

A specialised area that is thought to be the dock and release sites for synaptic vesicles

46
Q

What are the proteins involved in the vesicular release of neurotransmitters ?

A
  1. SNARE proteins
  2. MUNC 18
  3. Complexin
  4. Synaptotagmin
47
Q

What are the three SNARE proteins in the vesicular release of neurotransmitters ?

A
  1. Synaptobrevin- VAMP
  2. Syntaxin- 1
  3. SNAP-25
48
Q

What does complexin do ?

A

Prevents certain synaptotagmin-SNARE binding interactions

49
Q

What does synaptotagmin do ?

A

Cooperatively binds calcium and binds to both SNARE complex and phospholipid membrane to effect fusion pore opening

50
Q

What is the SNARE complex made up of ?

A

Four alpha helix bundles formed between the vesicle and cell membrane

51
Q

Where is synaptobrevin expressed ?

A

On the vesicle membrane

52
Q

Where is syntaxin-1 and SNAP-25 expressed ?

A

Cell membrane

53
Q

What is zippering ?

A

A process by which the alpha helices of the three SNARE proteins intertwine, resulting in the cell membrane effectively pulling the vesicle down to the membrane so the lipid bilayers are more tightly opposed

54
Q

What happens at sites in the alpha helix in the SNARE complex ?

A

Bacterial endopeptidases can cleave all proteins

55
Q

What does cleaving proteins in the SNARE complex achieve ?

A

Prevention of neurotransmission

56
Q

What is the key intracellular sensor of calcium concentration ?

A

Synaptotagmin

57
Q

Where is synaptotagmin superimposed ?

A

On the SNARE complex

58
Q

What are the domains that synaptotagmin has ?

A

C2A and C2B

59
Q

What is the function of C2A ?

A

Binds three calcium

60
Q

What is the function of C2B ?

A

Binds two calcium

61
Q

How does C2A and C2B bind ?

A

Cooperatively which increases the affinity for more calcium with each calcium bound

62
Q

In the primed pre-fusion state, where does synaptotagmin bind ?

A

To the cell membrane and to the trimeric snare complex

63
Q

What happens once calcium is bound to SNARE complex ?

A

Synaptotagmin can now bind the lipid membrane

64
Q

What does binding of synaptotagmin to lipid membrane facilitate ?

A

Fusion and distortion of the membrane and the release of the neurotransmitter with the formation of the fusion pored

65
Q

What are the steps involved in neurotransmitter release ?

A
  1. Sodium channels open
  2. Voltage changes - depolarisation
  3. Voltage dependent calcium channels open - calcium flows down concentration gradient
  4. Low affinity calcium sensors in active zone bind calcium
  5. Calcium dependent exocytosis
  6. Rapid turn off as calcium diffuses away from active zone

Cells to Organisms - Lectures (36 decks)

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