Flashcards in Gasotransmitters and mitochondrial function Deck (47):
What are the 3 endogenous gasotransmitters?
What is different about the gasotransmitters compared to other gases in the body ?
They are classified as gasotransmitters because they are generated by specific enzyme systems
- generated all the time in all cell types
- they all have relatively short half lives
What enzymes produce nitric oxide?
made by enzymes nNOS, iNOS or eNOS
NOS= nitric oxide synthase
What enzymes produce carbon monoxide?
It is an odourless gas produced by enzymes HO-1 or HO-2
What enzymes produce hydrogen sulphide?
It smells like rotten eggs and is generated by enzymes CBS, CSE(CGL) and 3MST
How is nitric oxide formed?
NOS forms NO but it uses many different cofactors such as haem, BH4, flavin, oxygen and NADPH to produce NO from L-arginine
How could you block the production of NO?
by using competitors of arginine
What are the 3 different forms of nitric oxide synthase (NOS)?
nNOS (NOS1): neuronal NOS- it is present in the nervous tissue and skeletal muscle (actually well distributed)- important for cell communication
iNOS (NOS2): inducible NOS- calcium insensitive- present in the immune system and cardiovascular system for immune defence against pathogens
eNOS (NOS3 or cNOS): endothelial NOS is present in the endothelium for vasodilation
the different names of NOS are due to their location
How is hydrogen sulphide formed?
There are numerous pathways used to generate hydrogen sulphide but they all derive from cysteine
CSE: cystathionine gamma-lyase
CBS: cystathionine beta synthetase
MPST (3MST): 3-mercaptopyruvate sulfurtransferase
CAT: cysteine aminotransferase
BOTH MPST and CAT are found in the mitochondria
How is carbon monoxide generated?
Produced by HO enzymes: heme oxygenase = HO1 or HO2
HO1= is an inducible enzyme, it is a heat shock protein
HO2= constitutively active
What happens to the numerous haem protein in mitochondria?
When they are degraded in the mitochondria they are recycled to HO
What can HO1/2 be rapidly reduced to ?
It can be reduced to a powerful antioxidant biliverdin which can be converted to bilirubin which is an antioxidant
What is important about the gasotransmitters?
All of these gases are able to interact with each other
- Hydrogen sulphide can stimulate CO production via Nrf2
- CO can inhibit hydrogen sulphide by inhibiting CSE
- CO can stimulate all forms of NO
- Hydrogen sulphide can stimulate eNOS to produce NO
At high concentrations what are gasotransmitters?
They are toxic at high concentrations
- >50% of poisonings worldwide are caused by CO- as it is odourless its difficult to know when it is present
How can gasotransmitters induce toxic effects?
They can bind to complex 4 and prevent respiration by inhibiting the electron transport chain
their effects at lower concentrations are much more interesting
What can NO be classified as?
It is a powerful post translational modifier
What are the actions of NO?
It can stimulate the production of guanyl cyclase leading to the production of cGMP which has many cellular affects by affecting ion channels, protein kinases and phosphadiesterases
It can also cause s-nitrosylation which causes the addition of NO to the SH of a cysteine residue producing SNO - s-nitrosylation is as powerful as phosphorylation causing changes in proteins
What are the many different processes that nitrosylation affects in the CNS?
Useful for controlling protein activity BUT it can go wrong !
- redox reactions
- cell death and injury
- transcriptional control
- mitochondrial function
- protein quality control: intracellular protein manipulation such as protein folding
- synaptic function
How can s-nitrosylation go wrong in AD?
- amyloid beta is a pathological hallmark of AD and it can disrupt astrocyte function causing activation of iNOS
- this occurs because activation of NMDA receptors occurs and this leads to too much calcium influx which activates iNOS
- leading to excessive nitrosylation
What can happen to DRP1 if nitrosylation goes wrong in neurodegenerative diseases?
DRP1 is involved in fusion/fission and it can be nitrosylated which can affect these processes
How does nitrosylation underlie excessive fission in AD?
1) nitrosylation of DRP1 leads to increased activity of DRP1
2) DRP1 protein is fundamental for zipping up the membrane during fission
3) mitochondria exposed to NO showed much more fragmentation and once complete fragmentation is reached it prevents the production of ATP
This is one process in which amyloid beta can cause degeneration in AD and cause the loss of synaptic connections
What effect did DRP-SNO (C644) have ?
It has the same effects as amyloid beta = neuronal loss and synaptic loss
How does nitrosylation modify mitochondrial respiration ?
it can modulate respiration directly
- NO can affect respiration at complex 4 causing it to slow down and it can also affect complex 1 causing it to slow down
This causes the ETC to slow which causes complex 1 to become leaky which leads to increased risk of superoxide production
How can the effects of too much s-nitrosylation at complex 1 be reversed?
reversed by glutathione
How does cardioprotection by NO occur?
s-nitrosylation fo a cysteine switch on complex 1 slows down its activity which is beneficial in I/R
- I/R is pathological process in which an organ is starved of oxygen and then the sudden reperfusion of it causes more damage which is thought to be due to reversal of the ETC
- in I/R injury there is a large release of ROS from complex 1 in reverse mode which can be slowed by nitrosylation and therefore is protective
What is metabolic syndrome?
involves obesity, hypertension, insulin resistance and hyperlipdemia
- it is potentially thought to be a predisposition to AD
What happens with eNOS becomes uncoupled?
it can form superoxide
TOO much superoxide can inhibit BH4 causing eNOS to be uncoupled leading to further production of superoxide (positive feedback)
- superoxide can also combine with NO producing peroxynitrite which is very damaging
What is hydrogen sulphide very important for ?
It is a very important regulator of cell function
- important for redox modulation- it increases GSH which is important for cytosolic protection and for preserving normal function - still unsure why it protects against oxidative stress
What is an important process that hydrogen sulphide can activate and how does it do this?
It can induce apoptosis but activating ERK1/2, p38 MAPK
therefore it can stimulate this process when it is required
What is s-sulfhydration ?
it is another process that induces post-translational modifications of proteins - it is a very powerful modifier of protein activity
- introduces another sulphur to cysteine residues which has multiple effects
What is an important property about the enzymes that produce hydrogen sulphide?
They either exist in or can be recruited to mitochondria
- the enzymes, CBS, 3-MST and CSE are important for mitochondrial function
What can hydrogen sulphide do to the ETC?
it can activate the sulphide oxidising unit which is present in-between complex 1 and 2
This unit oxidises sulphide, liberating electrons which can enter the ETC at complex 2 = beneficial for ATP production
What is the sulphide oxidising unit formed of?
sulphide quinone reductase
thiosulfate sulphur transferase
What are the other beneficial effects that hydrogen sulphide can induce?
it can inhibit calcium induced cytochrome c release and associated mitochondrial swelling which is known to be part of the damage in I/R injury
Reduced ROS production
What is the non-beneficial effect that hydrogen sulphide induces ?
It can stimulate GADPH which increases glycolysis which is an inefficient way of producing ATP and it can also initiate apoptosis
How has hydrogen sulphide been associated with neurodegeneration ?
AD: the decreased levels of CBS and the total decrease in hydrogen sulphide leads to less protection against inflammation and oxidative stress (ROS, pro inflammatory cytokines)
PD: CBS activity is inhibited and therefore there is decreased hydrogen sulphide leading to less protection against inflammation, oxidative stress, mitochondrial dysfunction and apoptosis
Hydrogen sulphide protects against loss of SN dopaminergic neurones via mitochondria
What are the actions of CO ?
CO is quite inert yet it affects many systems as it can interact with metals and any metal containing proteins
It is NOT involved in any specific chemical reaction but it is able to bind in a reversible manner to any metal or metal containing proteins which is a lot of molecules
- e.g it can affect NOS because it is metal containing
How can CO cause ROS formation/prevent it?
it can block complex 4 and this slows the electron transport system causing electrons to leak out
these electrons can react with oxygen leading to the production of superoxide
CO can inhibit ROS formation by NADPH oxidase
What effect does CO have on Nrf1/2?
It increases it expression, as well as PPARgamma1 mRNA
- this leads to mitochondrial biogenesis
increasing the levels of Nrf1/2 acts as a positive feeback loop because it will increase the production of CO
What effect does CO have on SOD2?
it can increase the amount of SOD2 which counteracts its ability to produce superoxide because this enzyme breaks down superoxide to hydrogen peroxide which is much less damaging
What effect does CO have on macrophages?
It can induce macrophage suppression which reduces inflammation and this subsequently reduces graft rejection- this is a means of CO becoming a therapeutic tool
What happens to HO-1 in an ischemic brain?
it is induced
- in a model where stroke was induced it demonstrated that the ipsilateral part of the brain which was exposed to chronic hypoxia demonstrated strong expression of HO-1 whereas the controlateral side and control brains didn't indicate expression of HO-1 or minimal
Is it beneficial that HO-1 is induced after stroke?
it is beneficial because when animals were exposed to relatively low levels of CO before blocking their cerebral blood flow you get much less damaged tissue therefore indicating that CO is a protective mechanism
What is the link between CO/HO1 in neurodegeneration ?
In a control aged brain there is some staining of HO-1 however in an AD brain there is substantial HO-1 staining
- this increased expression of HO1 is seen in PD and MS
What do a group n detroit think about the increased levels of HO1?
They think it is a bad thing and evokes damaging effects....
- think its increased expression in glial cells is bad as it liberates Fe2+ and this is involved in ferritin chemistry leading to hydroxyl radical formation which are as damaging as peroxynitrite
What does Prof Peers think about the increased expression of HO1 ?
thinks it is a good thing in neurones....
- used cell viability assays and exposed SH-5YSY (hippocampal neurones) to amyloid beta 1-42 which reduced viability and even more so in the presence of prion protein
when they overexposed HO1 there was significantly better survival upon exposure to amyloid beta
so they suggest that HO1 is providing protection by producing CO