Lecture IV Flashcards
Why does the mitochondria interact with the ER?
for division (or fission)
Why does the mitochondria interact with lysosomes?
Why does the mitochondria interact with peroxisomes?
Why does the mitochondria interact with the plasma membrane?
How have the mitochondria’s contacts been studies?
using imaging studies at a very high resolution
What mediates the contacts of the mitochondria?
tethers and proteins between the 2 organelles
What are the regions where the mitochondria overlap the ER called?
MAMs (mitochondria-associated membranes
What are the proteins that specifically link the ER and the mitochondria?
teethers
How were the tethers identified?
using electron microscopy
What is a protein recognized by MAMs?
mitofusin2 (Mfn2)
What is mitofusin2?
component of the OMM with promiscuous localization: it is on the OMM of mitochondria but also in the membrane of the ER
What does the MAMs mitofusin2 function as?
teether
What parts of the picture do we trust?
Mitofusin2 and the tripartite structure that includes the inositol-triphosphate receptor on the ER membrane
GRP75 chaperone
VDAC (porin)
Why are MAMs important for lipid biosynthesis?
they are essential to reach the final lipid composition of organelles
List all the enzymes important for lipid biogenesis:
MAMs
FACL4
ACAT1
What is the FACL4 (fatty acid-coA ligase 4) enzyme important for?
addition if acetyl-coA to fatty acids
What is the ACAT1 enzyme important for?
esterification of fatty acids so they can be metabolized by cytochrome P450 (1st step of steroid genesis)
List the calcium transfer pathway:
IP3 ligand binds to IP3 receptor on ER membrane → calcium is released →GRP75 allows calcium flux to be conveyed to the VDAC channel on OM → specificity is given by MCU
What is peculiar about MCU?
it is a uniporter located on the IM and it has LOW affinity for calcium
*MCU opens at the MAMs (since calcium conc. is very high due to the direct flux of calcium from the ER)
Why is it important for the calcium inside the mitochondria to be balanced?
many TCA cycle enzymes are calcium dependent so the calcium entrance is needed to maximize ATP production
What is a 3rd role of MAMs?
midzone fission
What are the 2 types of fission?
midzone fission
peripheral fission
What is midzone fission important for?
proliferation of the mitochondria
What is peripheral fission important for?
remove damaged parts
What is the interaction between the ER and mitochondria required for?
midzone fission
What are melanosomes?
organelles very similar to lysosomes and present in pigmented cells
What causes ocular albinism?
alteration of melanosome biogenesis
What are peroxisomes?
they contain a crystalloid core that contain many enzymes related to oxidated reactions
*this distinguishes them from lysosomes
Where are peroxisomes abundant?
tissues involved in lipidogenesis (liver and kidney)
How are fatty acids imported into the mitochondria?
through beta oxidation of fatty acids
How are fatty acids imported in peroxisomes?
through beta fatty acid oxidation (but with very long fatty acid chains with 22 or more carbons)
What is another process, apart from beta oxidation of very long fatty acids, that takes place in peroxisomes?
alpha oxidation of branched fatty acids
synthesis of bile
de novo lipogenesis (importing acetyl-coA)
What is PMP70 a transporter for?
both branched-chained fatty acids-coA and bile acid synthesis
What is synthesized in peroxisomes and is the key component of myelin sheath of neurons?
plasmologen
What is the aim of beta oxidation in mitochondria?
generate acetyl-coA to enter the TCA cycle and generate NADH to be used by OXPHOS to generate ATP
What is the aim of beta oxidation in peroxisomes?
generate hydrogen peroxide
*this means the beta oxidation of very long fatty acid chains generates ROS so peroxisomes need detoxifying enzymes
What is an enzyme devoted to the detoxification of ROS in peroxisomes?
catalase, which is part of the crystalline core of oxidases
How does the biogenesis of peroxisomes occur?
through the release of pre-peroxisomal vesicles from the ER r mitochondrial membrane
What do the pre-peroxisomal vesicles contain?
proteins like Pex3, Pex19, Pex16, Pex14, where “Pex” stands for “peroxisomal proteins”
How does a peroxisome become mature?
the pre-peroxisomal vesicles have to import proteins that contain a targeting sequence specific for organelles
How long are the target sequences of peroxisomes, mitochondria, and ER?
very short: SKL (serine, lysine, leucine)
How is the process of protein internalization of peroxisomes different from mitochondria?
Pex5 and Pex7 (proteins that are devoted to the import of C-terminal or N-terminal containing SKL sequences)are not integral to the membrane but can shuttle between peroxisome and cytosol
What is the step after biogenesis of peroxisomes?
growth and division
How does growth and division of peeroxisomes work?
Pex11 mediated an asymmetric elongation to form oligomers in order to generate protrusion
the protrusions present the same receptors for Drp1 as the mitochondria (like MFF or FIS1)
*this means Drp1 (mediator of mitochondrial fission) can also mediate peroxisomal fission
**this implies that even though peroxisomes can be generated de novo, they can also be undergo fission but only for fragments necessary in biogenesis
What is the average half-life of a peroxisome?
1-2 days
How are peroxisomes degraded physiologically?
via macro-autophagy
Describe the process known as pexophagy:
peroxisomes are engulfed by the autophagosome
autophagosome fuses with lysosome
the components are recycled
What makes pexophagy specific?
it involves 2 specific proteins: Pex2 and Pex5, which are located in the membrane of the peroxisome
Pex2 ubiquitinates Pex 5 and then binds to an autophagy receptor, NBR1 and p62 (sequestosome)
the proteins then bind to LC3, which belong to the autophagosome membrane
What are peroxisome related diseases related to?
severe diseases that affect the CNS
many times the disease is multi systemic because peroxisomes are necessary for lipogenesis, so many patients have kidney or lover problems
What is one of the most important disease related to peroxisomes?
Zellweger syndrome
What is Zellweger syndrome?
genetic heterogenous disease that can cause excessive mutations in any of the genes involved in peroxisome biogenesis
*severe disease and in most cases it is lethal in the 1st year of life
**at birth, babies have dysmorphic faces, problems in renal and liver functionality and neurodevelopmental problems
What causes X-linked adrenoleukodystrophy (X-ALD)?
mutations in the ANCD1 transporter, which is revelant for the very long chain fatty acid inside the organelle
*not lethal, but the patients present a very strong demyelination so the neurons do not function properly
What is interesting in regard to plasmalogen level in post-mortem Alzheimer’s patients?
lower level of plasmologen are found
What has been observed as a side effect of some chemotherpic agents (such as oxaliplatin or doxorubicin)?
it can lead to neurological and motor symptoms
How are peroxisomes and mitochondria interlinked?
they are interlinked functionally as they share part of their transcriptional control of biogenesis
Describe the 3 ways peroxisomes and mitochondria are interlinked:
through inter-organelle diffusion: ROS and lipids can be transferred directly from peroxisomes to mitochondria usinf inter-organellar diffusion
physical contacts mediated by a protein called Enoil-coA delta isomerase 2: N-terminal target sequence for mitochondrial import and C-terminal target sequence for peroxisome import (it works like a bridge and it is believed to maximize the process of ROS and lipid transfer between 2 organelles
vesicular traffic: mitochondria derived vesicles are necessary for mitochondrial biogenesis, but it is thought the vesicles can deliver peroxisomes fatty acids and specific proteins as well (the vesicles derive from the mitochondria and reach the peroxisome, and they are labelled with a specific protein called MAPL)
What are mitochondria and peroxisomes both involved in?
oxidative stress response
What happens when oxidative stress is present in the peroxisome?
mitochondrial enzyme activity is reduced → aconitase activity is reduced mitochondrial oxidative stress is increased → mitochondrial membrane potential is decreased → cell death occurs
What is Drp1 important for?
mediating the fission of both mitochondria and peroxisomes
What is Drp1?
cytosolic protein that contains a GTPase domain and a middle domain
What do Drp1 mutations linked to?
severe diseases in childhood (such as encephalopathies and deterioration of motor skills and sensory dysfunctions)
What did the D146N novel mutation in the GTPase domain of Drp1 result in?
most severe form of neurodevelopmental syndrome described in literature
What was found in regards to the novel mutation D146N in the GTPase domain of Drp1?
mutation acts as a dominant negative , which means that the mutant protein abolishes the function of the WT protein
*it is assembled into oligomers, but the GTPase activity of the oligomers is completely abolished
**this means the fusion process is not completed…NEVER
Why is the GTPase activity of Drp1 important?
it is the mediator of fission
a mutation in this is the reason for hyper elongated mitochondrial network
What are the consequences of the hyper elongation of the mitochondrial network resulting in balloons?
the balloons cannot be transported in neurons, so they stack in the soma
a problem then occurs in the turnover of the the damages part of the mitochondria
Where can mitochondrial derived vesicles (MDVs) bud from?
inner or outer membrane of the mitochondria in a Drp1 independent manner
What are the salvage pathways mentioned previously?
fusion (content of the membrane can be restored and ROS diluted)
peripheral fusion (eliminate damaged part)
reversal activity of ATP synthase
vesicular pathway (get rid of damaged material by engulfing this material inside the vesicle to deliver to lysosomes for degradation)
How many membranes do vesicles derived from the inner membrane have?
2
ow many membranes do the vesicles derived from the outer membrane have?
1
What distinguishes the OM and IM vesicles?
not only the number of membranes, but also their surface markers
TOM20 is found on vesicles directed to the lysosomes
MAPL is found on vesicles targeted to the peroxisomes
Describe the picture:
mitochondrial derived vesicles have been associated with innate immunity
ER stress caused by streptococcus infection enhances peroxide production inside the mitochondria
budding of microvesicles containing peroxides and Sod2 occurs
the vesicles are internalized by the phagosome, the peroxide, and the Sod2 enzyme, which further generates peroxide and kills the bacterium
