Lecture 6 - Regulation of rapid cellular events (regulated exocytosis)

Question 1

a critical part of cell to cell communication is

Answer 1

the release of signalling molecules

Question 2

Lipid soluble molecule release

Answer 2

Lipid soluble molecules are released by diffusion as soon as they are synthesised (across the synapse, extracellular space etc)

Question 3

water soluble molecule release

Answer 3

Water soluble molecules are released from vesicles via regulated exocytosis (signalling molecules are packaged into vesicle molecules and then fuse with the membrane to release its contents)

Question 4

vesicles generated by

Answer 4

Vesicles are generated by the Golgi and tagged to specific destinations

These include the cell surface where they release their contents by exocytosis

Question 5

constitutive exocytosis

Answer 5

Constitutive - extracellular matrix materials are the ones transported, the vesicle just empties

Question 6

regulated exocytosis

Answer 6

Regulated - signals (paracrine, endocrine and neuronal) vesicle does not release until it is told to, want signalling cell to send the correct message at the right time

Question 7

vesicles

Answer 7

Has a phospholipid bilayer
Water soluble molecules are trapped inside
Hydrophilic head and hydrophobic tail

There are multiple vesicle associated proteins whose functions include:
Building the vesicle
Loading of signal molecules into the vesicle
Guiding the vesicle to the correct location
Regulating exocytosis
Vesicle recovery after exocytosis

All sorts of proteins that are buried in the lipid membrane or stuck on the outside - molecular machinery

Question 8

Key steps in regulated exocytosis (2)

Answer 8

vesicles must find the right release site and avoid the wrong ones

getting a vesicle to the correct site involves a number of proteins in addition to the SNAREs

Question 9

Key steps in regulated exocytosis - vesicles must find the right release site and avoid the wrong ones

Answer 9

Vesicles must find the right release site and avoid the wrong ones
Interaction is mediated by SNARE proteins
v-SNARE on vesicle
t-SNARE on target
V- and t-SNARES form complementary pairs
A SNARE complex will lock the vesicle at its correct location
Vesicle trafficking occurs throughout the cell
Best studied at the nerve terminal
Golgi is the location where vesicles are made and tagged with a protein called the v-SNARE and the job for it is too find a matching t-SNARE which ensures that vesicles go to the right place
Syntacin and Snap25 are t-SNAREs and there is two target SNAREs to ensure that the vesicle is correct for that location
Synaptobrevin is the v-SNARE

Question 10

Key steps in regulated exocytosis - getting a vesicle to the correct site involves a number of proteins in addition to the SNAREs

Answer 10

Getting a vesicle to the correct site involves a number of proteins in addition to the SNAREs
Including the multiple proteins that control exocytosis
Central to this event is synaptotagmin which makes release Ca2+-dependent - it is important because it is the one that is able to sense calcium/designed to bind the calcium, when calcium binds to it changes its shape which allows for the vesicle to find a piece of membrane to fuse to
Calcium binding to synaptotagmin changes its shape bringing the vesicle towards the cell membrane
Thus for regulated exocytosis to occur a cell must increase its calcium concentration at the site of release
How does this occur in specific cells and what role do receptors play
When the vesicle gets to the synaptic membrane, the v-SNARE finds the corresponding t-SNARE and it leads to fusion on the cell that wants the signal

Question 11

Summary of steps for regulated exocytosis - headings

Answer 11

tethering
docking
fusion

Question 12

Summary of steps for regulated exocytosis

Answer 12

Tethering - Upon reaching the cell membrane, the vesicle becomes linked to and pulled into contact with the cell membrane.
Docking - involves the attachment of the vesicle membrane with the cell membrane. The phospholipid bilayers of the vesicle membrane and cell membrane begin to merge.
Fusion - complete fusion where membrane completely fuses, kiss and run where the vesicle temporarily fuses with the membrane

Question 13

Importance of calcium in regulated exocytosis

Answer 13

A rapid increase in intracellular calcium directly triggers regulated exocytosis

Question 14

Examples of exocytosis in adrenal chromaffin cell

Answer 14

Found within the adrenal medulla
Releases catecholamines (adrenaline and noradrenaline) in response to stress
Known as the fight or flight response
Secretion stimulated by acetylcholine release from nerve
Activates nicotine acetylcholine receptor (when ACh binds it opens)
Receptor channel opens and sodium influxes
Cell depolarises and opens voltage gated calcium channels
Ca2+ influx causes exocytosis
Ca2+ can also stimulate tyrosine hydroxylase (TH) to make replacement catecholamines (and this is in case the stress isn’t over

Question 15

Examples of exocytosis in pancreatic beta cell

Answer 15

Found within the islets of langerhans
Release insulin in response to increased glucose concentration in the blood (insulin takes glucose out of your blood and puts it into cells)

Glucose is transported into the beta cell
GLUT are glucose transporters and this job is to take glucose from the blood and put it into the cell and will be metabolised to make ATP

Metabolised to form ATP
ATP acts to close a K+ channel, when formed the ATP binds to the K+ channels and therefore potassium will flow out with its concentration gradient but the ATP stops it leaving which ends up changing the voltage inside the cell, stopped positive charge leaving therefore depolarisation is occurring
More positive within the cell causes it to depolarise
Opens voltage-gated Ca2+-channels
Ca2+ enters the cell
Stimulates exocytotic release of insulin
Also increases insulin synthesis - there is a mechanism for releasing insulin and for making more
Negative feedback is the blood glucose concentration that these cells detect
Insulin acts on insulin receptors (on liver, muscle, brain, adipose) to remove glucose from the blood into cells

Question 16

Examples of exocytosis in mast cell

Answer 16

Cells of the immune system found within connective tissues
Contain multiple granules (vesicles)
Undergo massive exocytosis called degranulation
Release a cocktail of biologically active molecules
Provide the first-line of defence to foreign antigens, tissue damage and pathogens (detects microbial attack and responds)
Allergens cross-link two IgE (FcεR1) receptors - brings two of them together and it causes degranulation
Triggers a pathway that releases calcium from the endoplasmic reticulum
Increased calcium causes degranulation
Receptor activation also activates other intracellular pathways that increase production of more cytokines

Question 17

Examples of exocytosis in neuron/synaptic terminal

Answer 17

Neurotransmitters are released from vesicles in the nerve terminal
Arrival of an action potential causes the nerve terminal to depolarise via opened sodium channels (voltage gated)
And voltage gated Ca2+ channels open
These channels are positioned close to the release sites
Ca2+-influx stimulates vesicle exocytosis
And thus release of neurotransmitter
Which acts on post-synaptic receptors

What is the role of receptors in this process?
Generation of an action potential results from the sum of multiple stimulatory or inhibitory inputs on the dendrites and cell body of the transmitting neuron
These are mediated by receptors
The post synaptic target cell expresses receptors for the released transmitter
There are also receptors on the presynaptic terminals

What is their function?
Signals to the presynapse may provide feedback to turn off further release
Can be mediated by the neurotransmitter
1= can be mediated by the neurotransmitter
2= a signal can also be sent from the post-synaptic neuron
Endocannabinoids (from cannibis) which are made in the post-synaptic terminal in response to an increase in Ca2+ feedback on the nerve terminal via a GPCR to reduce further neurotransmitter release (negative feedback system)

Question 18

Importance of negative feedback in cell to cell communication

Answer 18

Negative feedback serves to dampen and/ or limit signalling.

Vital control mechanism for the body’s homeostasis