Lecture #4 Flashcards
what are MAMs?
mitochondrial-associated membranes; specifically for mito-ER contacts
describe the contact between two organelles:
strictly associated but never fused
what is the main player recognized in MAMs
mitofusion 2
where is mitofusion 2 located?
in the omm as well as in the ER membrane
where is mitofusion 1 located?
only in the omm
what is the function of mitofusion 2?
acts as a tether
what is FACL4?
a fatty acid-coA ligase4 - mediates the addition of acetyl-CoA to fatty acids
what is the role of MAMs in lipid biosynthesis?
essential in reaching the final lipid composition of organelles
what is the role of ACAT1 enzyme?
important for the esterification of fatty acids
what is the first step of steroidogenesis?
the presence of esterified cholesterols imported in the mitochondrial inner membrane where they are metabolized by cytochrome p450
what does the first step of steroidogenesis also serve as?
the rate limiting step for the process
what is the true specificity for calcium transfer given by?
the MCU
what is the most peculiar feature of the MCU?
it is a uniporter
where is the MCU located and what is its main characteristic?
the inner mitochondrial membrane - has a low affinity for calcium
where does the MCU open?
at the MAMs
why does the MCU open at MAMs?
in those sites calcium concentration is very high due to the influx of calcium from the ER to the mitochondria
why must the calcium levels in the mitochondria be balanced?
many TCA cycle enzymes are calcium-dependent and so the calcium entrance is needed to maximize the ATP production
describe the two types of permeability transition pores in the imm:
low conductance or high conductance
what happens in low conductance pores?
they can release precise amounts of calcium → flickering: small waves of calcium that are released to maintain the desired concentration
what happens in the high conductance pores?
triggered by mitochondrial calcium overload and is very dangerous → causes permeability transition in which it alters the permeability of the imm creating a bidirectional flux of solutes / ions / water eventually triggering apoptosis
what is the third role of the MAMs?
mid-zone fission
what are the two types of fission used by MAMs?
midzone fission (required for proliferation) and peripheral fission (required to remove damaged parts)
what was another connection between mitochondria discovered by scientist at UniSR?
contact between mitochondria and melanosomes → mediated by fibrillary bridged composed of mitofusin II - seems to be important for melanosome biogenesis
where are peroxisomes most commonly found?
found in all cell types but very abundant in this tissues involved in lipogenesis → particularly in the liver and kidneys
describe the composition of peroxisomes:
present a single membrane that is quite permeable, however there are specific transporters in the membrane for the import of fatty acids and other kinds of lipids, for the import of proteins and for the import of acetyl-CoA
what type of fatty acids do peroxisomes deal with?
VLCFA
what is synthesized in peroxisomes that is a key component of myelin sheaths of neurons?
plasmalogen
what clinical presentations do the mutation of Peroxisomes often cause?
strongly affected the CNS and PNS
in the mitochondria, what is the aim of beta oxidation?
to generate acetyl-CoA which enters the TCA cycle
in the peroxisomes there aren’t OSPHOS, so what happens to the high potential electrons generated during beta oxidation?
transferred to molecular oxygen in order to generate hydrogen peroxide
what is the beta oxidation of VLCFAs in peroxisomes closely coupled with?
tightly coupled to ROS production
what must we have inside of peroxisomes because of the way they deal with VLCFAs?
a lot of enzymes devoted to the detoxification of ROS → catalase (part of the crystalline core of oxidases)
what causes peroxisome biogenesis to occur?
the release of pre-peroxisomal vesicles from the ER or mitochondria membrane
what does PEX stand for?
peroxisomal proteins - essential proteins for peroxisomal biogenesis
what must occur for a vesicle to be imported selectively in a specific organelle?
a protein must contain a targeting sequence which is different based on the organelle
describe the import sequence:
only three aa that can be located in either the C or N terminal
what peroxisomal proteins are devoted to the import of of c-terminal or n-terminal containing SKL sequences?
Pex 5 and Pex 7
describe the action of Pex 5 and Pex 7:
not integral to the membrane but can shuttle between peroxisomes and the cytosol → bind proteins in the cytosol and deliver them to the peroxisomes, releasing the inside the lumen - then they can be transported back to the cytosol
what is another step of peroxisome biogenesis besides import?
growth and division
what is the asymmetric elongation mediated by, and what does it form?
mediated by Pex 11, and forms oligomers in order to generate a protrusion → these protrusions present the same receptor for Drp1, which can mediate peroxisomal fission
what is different between peroxisomes and mitochondria?
peroxisomes can be generated de novo
what players do mitochondrial and peroxisomes share?
same mechanisms of fission but peroxisomes cannot fuse but only fragment
what is the half life of peroxisomes?
1-2 days
what is pexophagy?
peroxisomes are engulfed by the autophagosome which then fuse with the lysosome allowing the degradation of the peroxisomes and the recycling of its components
what two specific proteins are involved in pexophagy?
Pex2 and Pex 5
in pexophagy, once Pex 2 ubiquinates Pex 5, what occurs?
similar to PINK / Parkin → there is the binding of the specific autophagy receptor NBR1 and the binding of p62 (aka sequestosome) - in turn these proteins bind to LC3 which are the proteins belonging to the autophagosome membrane
generally describe peroxisome-related disease:
very severe, mostly affecting the CNS but in many cases are multi-systemic disease
what is one of the most prominent disease of the peroxisomes?
Zellweger syndrome
what is Zellweger syndrome?
a genetic heterogenous disease that can be caused by recessive mutations in any of the genes involved in peroxisome biogenesis → very severe and lethal in the first year of life
what are the clinical signs of Zellweger syndrome at birth?
dysmorphic face, problems with renal and liver function, and neurodevelopment delays
what is an example of a single protein peroxisome defect?
x-linked adrenoleukodystrophy
what is x-linked adrenoleukodystrophy?
caused by mutations in the ABCD1 transporter which is relevant for VLCFA inside the organelle
what is the clinical presentation of X-ALD?
not lethal, but patients present a very strong demyelination and so neurons do not function properly
primary peroxisome defects have been found in what kinds of disease?
degenerative and age-related conditions of the CNS
how are peroxisomes and mitochondrial functionally interlinked?
the share part of their transcriptional control of biogenesis
what are the three main players of transcription in mitochondria?
TFAM (transcription factor for transcribing all mitochondrial genes), NFR1&2 (transcription factos important for genes encoding components of the respiratory chain), and PPAR⍺
TFAM, NRF1&2, and PPAR⍺ are all part of what family?
the PPAR⍺ family → shared between mitochondria and peroxisomes because they enhance transcription of genes implicated in the metabolism of fatty acids
besides the PPAR⍺ family, what other regulator do mitochondria and peroxisomes share?
PGC1-⍺ → master regulator for all those pathways
what three ways are mitochondria and peroxisomes known to communicate?
- inter-organellar diffusion
- can make physical contacts - “bridge”
- vesicular traffic
in what way do mitochondria and peroxisomes communicate through inter-organellar diffusion?
it has been reported that ROS and lipids can be transferred directly by inter-organellar diffusion from peroxisomes to mitochondria and vice versa
in what way do mitochondria and peroxisomes communicate through physical contacts?
it has recently been demonstrated that these two organelles can make physical contacts mediated by a protein called Enoil-CoA delta isomerase 2 → works like a bridge between the two organelles and this physical interaction is believed to maximize the process of ROS and lipid transfer between the organelles
in what way do mitochondria and peroxisomes communicate through vesicular transport?
mediated by mitochondrial-derived vesicles (necessary for mitochondria biogenesis) → the vesicles that bud from mitochondria and reach the peroxisome are labelled with a specific protein of the mitochondria outer membrane called MAPL
what occurs in aging and neurodegenerative conditions related to levels of ROS?
there is an increase in ROS production that is not counterbalanced by ROS scavenging, so this generates an accumulation of ROS and generates strong oxidative stress
what can oxidative stress present in peroxisomes cause in mitochondria?
it can reduce mitochondrial enzyme activity, causing:
- a reduction in aconitase activity
- an increase in mitochondrial oxidative stress
- a reduction in the mitochondria membrane potential
- finally leading to cell death
what is Drp1?
a cytosolic protein belonging to the dynamine protein superfamily, it contains a GTP-ase domain and a middle domain
what de novo mutation was discovered in Drp1?
D146N in the GTPase domain of Drp1
what does the D146N mutation form in patients?
balloons → hyperelongation that cannot be transported in neurons causing them to stack up in the soma
what type of mutation is D146N?
dominant negative → since the mutant protein is in the heterozygous state, it abolishes the wild type function of the protein - it is recruited and assembled into oligomers, but the GTPase activity of the oligomer is completely abolished = fusion is never completed
what are the result of the balloons?
- they aren’t transported
- there is a problem in the turnover of the damaged part of the mitochondria - both in peripheral fission and mitophagy because the organelle is too big to be engulfed by the autophagosome
what is a mito-timer?
fluorescent protein that can be delivered to mitochondria and the fluorescence of this protein changes depending on protein maturation
what is the role of doxycycline with a mito-timer?
binds to the TET promotor to drive the expression of that protein in a precise moment ⇢ can be synchronized
what are mitochondria derived vesicles (MDVs)?
can bud from the inner or outer membrane in a Drp1 independent manner (not fission)
what is the effect on budding if we silence Drp1?
the bidding of MDVs are increased, furthermore demonstrating that this process is Drp1 independent
what do MDVs require instead of Drp1?
require PINK1-Parkin in a localized way
what are the two different types of vesicles?
those directed to the peroxisomes and those directed to lysosomes
what are the vesicles directed towards peroxisomes needed for?
needed for peroxisomal biogenesis or for the transfer of oxidized proteins or ROS to peroxisomes
what is the role of the lysosome targeted micro vesicles?
another salvage pathways that mitochondria put in place in order to ge rid of the damaged material without triggering whole organellar mitophagy
what are some mitochondrial salvage pathways?
fusion, peripheral fission, reversal activity of ATP synthase, vesicular pathway
if these vesicles are derived from the inner membrane, do they present a double membrane?
there is a double membrane in vesicles derived from the inner membrane and a single membrane in the vesicles derived from the outer membrane - what distinguish them is the surface marker: Tom20 for those vesicles that are directed to the lysosomes, while for those that are targeted to the peroxisomes, we have the specific protein called MAPL
what is an example of how mitochondrial derived vesicles have been associated with innate immunity?
macrophages were infected with streptococcus aureus (infection that causes ER stress and enhanced peroxide production in mitochondria) → after there was a budding of micro vesicles containing peroxides and even the enzyme implicated in the generation of peroxides - these vesicles were delivered ot the phagosome in order to kill the bacterium
What are the three roles of MAMs?
Lipid biosynthesis, calcium transfer, midzone fission
