Lecture 14 & 15: exocytosis Flashcards
What is exocytosis?
- a form of active transport in which a cell transports molecules (eg neurotransmitters or hormones) out of the cell
what is the 2 step process for exocytosis?
- docking
- fusion
describe the 2 types of pathways that are used for exocytosis
-
Constitutive secretory pathway
* in all cells,** transport vesicles leave the trans golgi network (TGN) and go to the plasma membrane **
* they secretory vesicles provide new components for the plasma membrane and components that are designed for secretion eg proteoglycans and glycoproteins of the extracellular matrix -
regulated secretory pathway
* specialised secetory cells use a 2nd pathway - involves proteins stored in secretory vesicles for release at a later stage
How are proteins leaving the golgi network sorted into 3 classes?
they are sorted into 3 classes based on their destination
Describe the 3 classes of proteins (based on their destination)
- mannose 6 phosphate (M6P) is used as a sorting signal to direct proteins to lysosomes
- some proteins in specialised secretory cells direct them to the** regulated secretory pathway** (via secretory vesicles)
- the** constitutive pathway** is used to deliver proteins that lack specialised signals
Describe the regulatory secretory pathway
- specialised cells store their products in secretory vesicles that are involved in the regulatory secretory pathway
- proteins aggregate in the ionic environment of the trans golgi network and condense further as the vesicles become more acidic and mature
- the signal for the initiation of the regulated secretory pathway **involves the binding of a hormone or neurotransmitter to a receptor on membrane ** which causes the cell to exocytose the secretory products
what signal ** initiates** the regulated secretory pathway?
- the binding of a neurotransmitter or hormone onto a receptor of the plasma membrane
What is the ‘full collapse’ method of secretion?
- it was previously thought that secretory vesicles completely merge with the plasma membrane before releasing content
- the vesicles can then be endocytosed after they release their content
what is the ‘kiss and run’ secretion method for secretory vesicles?
- vesicles can quickly dock and expel their contents without full fusion to the plasma membrane
- vesicles can then be refilled so each vesicle can undergo multiple rounds of secretion
Describe the kiss and run method for synaptic vesicles
- synaptic vesicles turn over rapidly, instead of returning back to the endosome and cell body - they are refilled with neurotransmitter
what are porosomes?
- they are cupshaped lipoproteins found in the plasma membrane of eukaryotic cells
- they are the sites** where the secretory vesicles transiently dock** in the process of vesicle fusion and secretion
Describe what happens to the secretory vesicle at the porosome
- the synaptic vesicles dock at the porosome base which develops intravesicular pressure (swell) via the active transport of water through aquaporins
- the vesicles transiently fuse at the porosome base via the snare proteins and Ca2+
- they then expel their contents (eg neurotransmitters)
Describe the life cycle of a neurotransmitter
- they are released from the presynaptic nerve terminal
- they diffuse across the synaptic cleft
- they bind to receptors on the post synaptic membrane (eg glutamate binds to AMPA)
- they are then broken down by enzymes or reuptake by transporters to neurons and glia
what 2 key proteins are involved in vesicle filling?
- the vacuolar proton pump
- neurotransmitter- specific vesicular transporter
what is the role of the vaculolar pump in vesicle filling?
hint : ATP
- it is a multi unit ATPase
- it creates** transmembrane electrochemical gradient **
- the electrochemical gradient acts as a energy source for active uptake of transmitter by transporter proteins
what is the role of the neurotransmitter specific vesicular transporter?
- it is an integral membrane protein that transports neurotransmitters
- 4 types have been identified - one for Ach, one for catecholmines & serotonin, one for glutamate and one for GABA
How does** Ca2+ affect the release** probability ?
- the synaptic vesicles are held in place by Ca2+ sensitive membrane vesicle proteins (VAMPS) eg synapsin I
- during an AP, intracellular Ca2+ in the axon terminal can rise significantly
- **elevated Ca2+ levels activate Ca2+-dependent calmodulin kinase **(CAMKII)
- CAMKII phosphorylates synapsin I which releases the vesicle from the plasma membrane
what is an example of a Ca2+ sensitive vesicle membrane protein (VAMP)?
- synapsin I
what are examples of proteins involved in vesicle release?
- SNARE proteins - eg synaptobrevin is bound to the vesicle membrane
- syntaxin and SNAP25 are bound to the plasma membrane
- synaptotagmin - calcium binding protein- when Ca2+ binds to it, the protein binds to the SNARE proteins
what is vesicle priming?
it is the process that synaptic vesicles must undergo after docking and before fusion
what are the main SNARE proteins in the snare complex?
- **VAMPS **(vesicle associated membrane proteins) - eg synaptotagmin & synaptobrevin
- syntaxin - protein on the plasma membrane
- SNAP25 - 2 protein strands that bring the vesicle membrane and the plasma membrane together
what happens to the SNARE complex after fusion?
the snare complex **disassembles **
what are examples of pathological** disorders of the NMJ**?
- Myasthenia Gravis
- Lambert Eaton syndrome
- Tetanus toxin
- Botulinium Toxin
what kind of disease of myasthenia gravis?
- an auto-immune disease
- antibodies are produced that are directed against the Ach receptors at the NMJ which decreases the availibility of receptors for binding to Ach
*
what are the** symptoms** of myasthenia gravis?
- long term skeletal muscle weakness
- most common areas affected are the** eye and eyelid movement, facial expression** &chewing, talking and swallowing
- more progessive forms can target the respiratory tract, neck and limb movement
how could MG symptoms possibly be reversed?
- by using inhibitors of AChe
what **structural changes **does MG cause in the NMJ?
1.** enlargement** of the synaptic cleft
2. junctional folding is reduced
Describe the **mechanism of action **of MG
ie how do the antibodies interact with the receptors?
- the antibodies may bind and cross-link the receptors
- cross linking of the antibodies to the receptors sends a signal to speed up decomposition by phagocytosis
- they may trigger receptor recycling by endocytosis
- lysosomes also involved to help with decomposition
Describe the possible causes of MG?
- autoimmune disease
- bacterial infection- bacterial antigens may share epitopes with the Ach receptors
- Viral infection -a viral protein may include a peptide sequence that is similar to one in the Ach receptor
- thymomas- tumours derived from epithelial cells in the thymus - 50 % of patients with these tumours develop MG
what are the possible treatments for MG?
- there is** no known cure**
- lifestyle changes - ie increasing rest periods can help to restore function
* immunosupressant treatment - reduce and minimize the immune response (ie antibody production) - surgical intervention - eg if thymoma was the cause of MG
what type of disease is Lambert-Eaton syndrome?
- very rare autoimmune disorder
what are the** symptoms** for Lambert-Eaton syndrome?
- muscle weakness in limbs- note has greater impact to legs
- difficulty in climbing stairs and stand - sit movements
- difficulty walking
- in advanced stages of the disease, there can be respiratory difficulties
what is the mechansim of action of Lambert-Eaton syndrome?
- antibodies are produced that act against the P/Q type Ca2+ channels in the presynaptic which inhibits neurotransmitter release
- they bind partcularly to the doman III S5-S6 linker peptide
- this binding reduces the influx of Ca2+ to the presynaptic terminal
what are the possible treatments for L-E-S?
- no known cure
- immunosupressant treatment- less effective than for other immune conditions eg MG
- lifestyle changes - aimed at minimising risk of small cell lung cancer eg no smoking,reduce exposure to radiation
what are examples of pharmacuetical drugs that can promote the function of the NMJ?
*** Amifampridine **- only approved drug treatment for L-E-S, it blocks voltage gated K+ channels and prevents K+ efflux which prolongs the depolarisation and lengthens the action potential duration
What is tetanus caused by?
- tetanus is caused by the**tetanus neurotoxin **that is released from infection by Clostridium tetani
- the endospores of the bacteria live in an anaerobic environment eg soil, manure etc until they find a suitable environment to metabolise and proliferate
what are** examples of symptoms** of tetanus?
- symptoms can take up to 3 weeks to occur
- spasmogenic toxicity
- begins with the facial muscles - eg lock jaw (muscles of the jaw spasm) and in severe cases it can spread to other areas in the body
How does the tetanus toxin enter the NS and how does it travel?
- tetanus toxin enters the nervous system at the NMJ
- it migrates through the nerve trunks and into the CNS by **retrograde axonal transport **
- then there is** transcytosis** (transport across the interior of a cell) from the axon into the inhibitory interneurons of the spinal chord
what is the mechanism of the actual neurotoxin released in tetanus?
ie what transmitter does the neurotoxin interact with?
- the toxin acts by specifically interfering with glycine release from interneurons in the spinal chord
- glycine is an **inhibitory transmitter **in the spinal chord which acts on the glycine receptor - which is structurally and functionally similar to the GABA receptor
- if glycine release is blocked, there is an increase in skeletal muscle activity (ie spasm)
what are the treatments for tetanus?
- immunisation with the** tetanus vaccination** - for prevention
* tetanus antibody Iv infusion or im injection - antibiotics might reduce toxin production
- muscle relaxants to control spasms
What is **botulism **caused by?
- caused by the botulinum toxin that is a neurotoxin thats released by Clostridium botulinum
- the bacteria is mostly found in untreated water or** badly preserved food** eg tinned food
- can also be through wound infection or intestinal (ie bacterial spores develop in intestines and release toxin)
what are the symptoms of botulism?
- begins with muscle weakness
- blurred vision, speech disruption
- fatigue
- paralysis
- respiratory failure in severe cases
what is the **mechanism **of the botulinum toxin?
- there are** eight different toxins** (BT-A to G)
- botulinum toxin contains heavy and light chains
- the heavy chain binds to glycoprotein structures that are specifically found on cholinergic nerve terminals
- after internalisation, the** light chain** of the toxin binds with high affinity to the SNARE complex eg BT-A cleaves SNAP25
- this **prevents the docking of the Ach vesicle **and therefore **prevents vesicle fusion **and Ach release
what are examples of treatments of botulism?
- The key is early diagnosis
- if it was** food borne** - can treat with induced vomiting to remove contaminated food from gut
-
wound infection- can use surgery to remove the infected material from wound
* Botulinum antitoxin- removes toxin from blood stream to prevent worsensing of symptoms - antibiotics to treat wound infection
ewhat are the** therapeutic uses** of the botulinum toxin?
- treatment of disorders with** overactive muscle movement** eg cerebral palsy
- treatment of** excess sweating or salivation**
- migraines
