Lecture V Flashcards
What can elicit a very strong immune response?
vesicles released as exosomes to be taken up by recipient cells involved in innate immunity
What are DMAPs?
Damage Associated Molecular Patterns
What are the components of the mitochondria that are highly immunogenic?
inner membrane lipid cardiolipin
cytochrome c
mitochondrial DNA
What are the components of the mitochondria (inner membrane cardiolipin, cytochrome c, and mitochondrial DNA) recognized as?
DAMPs from the receptors of the macrophages or neutrophils and they elicit a strong immune response
Can a whole mitochondria fit in an exosome?
no
How can microglia or astrocytes provide healthy mitochondria to neurons?
via horizontal transfer mediated by microvesicles
How can neurons deliver damaged mitochondria to astrocytes?
via transmitophagy
What is a result of microglial cells releasing DMAPs? What is it associated with?
neurons are further damages
chronic neuroinflammation and neurodegeneration
What is mitostasis?
specialized form of homeostasis by which the mitochondrial number and quality are maintained over time
What are 2 QC levels of mitostasis?
molecular QC
organellar QC
What is molecular QC of mitostasis based on?
the control of the proteome inside the mitochondria
What is the organellar QC of mitostasis based on?
mitochondrial dynamics
What are the players of fusion and fission in mitochondrial dynamics?
Drp1 for fission
MFN1 and MFN2 (OMM) and OPA1 (IMM) for fusion
What are the 2 types of mitochondrial transport in neurons?
long range transport
short range transport
What is long range transport of mitochondria in neurons?
occurs in axons and based on microtubules and molecular motors that can mediate the anterograde transport and retrograde transport of mitochondria
What is short range transport of mitochondria in neurons?
extremely slower compared to neurons, and it is where the mitochondria are bound on the cytoskeleton by different motor proteins (ex: myosin)
What state do the mitochondria need to be in in order to be transported?
intermediate state, not completely fragmented, where there are short tubules
What are 2 crucial events important for neurons?
ATP production
calcium buffering
What are 3 challenges neurons face?
the need for sufficient supply as the ATP that comes from glycolysis is less than 10%, so 90% is provided by mitochondria
long distance of the mitochondria to reach the soma
local demand matching: synapses have highest need for ATP and calcium buffering, unmyelinated axons, at the nodes of Ranvier, also need a lot of mitochondria to control ion fluxes and maintain saltatory conduction
What are the main player of mitophagy?
PINK and Parkin
Where are PINK and Parkin translated?
within the axons
*this implies that the mRNA travel along the axon to be translated locally and it is driven by the local needs of the mitochondria
Describe the mitochondria in the axons of the CNS:
they are stationary
Describe axonal transport of the mitochondria:
transport is bidirectional: mitochondria moves both retrogradely and anterogradely
What percentage of mitochondria are motile in neurons?
only 20-30%
What is the “Kymograph analysis”?
a way to track the mitochondria as they move along the axons
*vertical bars indicate stationary mitochondrion
**oblique bars indicate anterograde or retrograde movements
What mediated the anterograde movement of mitochondria in axons?
kinesins
What mediates the retrograde movement of mitochondria in axons?
dyneins
What is anterograde transport?
transport from cell body to axon
What is retrograde transport?
transport from axon to cell body
How are kinesins, dyneins and myosins similar?
they all have a:
head domain: interacts with the microtubules as these motors walk along the microtubules
tail domain: interacts with the the cargo, the mitochondria
Describe what is important in regards to Kinesins:
3 types of kinesins in mammals
1 type is ubiquitously expressed: transport of mitochondria in every type of cell
KIF5A is most important in neurons
Describe what is important in regards to Dyneins:
they mediate retrograde movement towards the minus (-) end of microtubules
allow long-distance transport of mitochondria and other organelles on microtubules through mechanisms requiring ATP hydrolysis
Describe what is important in regards to myosin:
transport mitochondria along actin filaments
Myosin V is a motor protein and a possible transporter of mitochondria on actin filaments, which are enriched in synaptic terminals and in distal dendrites and responsible for mitochondrial docking
How are mitochondria attached to microtubules?
via kinesins
Once the mitochondria reach the terminals, how do they attach to microfilaments?
they detach from the microtubule and attach to microfilaments via Myosin V
What is Miro1?
integral calcium-binding protein of the OM of the mitochondria that contains 2 EF and GTP domains
What happens when Miro1 interacts with Milton?
it forms the Miro/Milton complex, which interacts with kinesins and dyneins so that mitochondria can be transported
How is the mitochondrial transport using the Miro/Milton complex tightly regulated in neurons?
calcium binds to the EF ends of Miro1 and this induces a conformational change in the complex by which the kinesin is detached from the microtubules, trapped by the Miro/Miltn complex
What is the consequence of Miro-Milton tightly regulating mitochondrial transport?
the mitochondria no longer move, so they exert calcium buffering
What does the protein Syntaphilin do?
anchors the mitochondria to the microtubules and stops them there
How are all stationary mitochondria bound to microtubules?
via Syntaphilin
What does the depolarization of the mitochondria induce the activation of?
PINk1-Parking pathway
What are targets of the PINK1-Parking pathway?
OM proteins that are ubiquitinated, such as Miro1 if the mitochondrion is damaged or mutofusins
How can mitophagy occur in the distal regions of the axon?
PINK1 and Parkin are locally synthesized by the ribosomes to allow mitophagy in distal regions
What kind of neurons show all aspects of mitostasis to be highly relevant?
purkinje neurons
What kind of neurons are Purkinje neurons?
GABAergic neurons, with inhibitory functions that make synapses with the deep cerebellar nuclei, which are in contact with the spinal neurons
What kind of control do Purkinje neurons exhibit?
fine control of movement and coordination
What is key to Purkinje neuron health and function>
mitostasis strictly correlated to the calcium homeostasis pathway, which needs to be finely controlled
What does the impairment of mitostasis result in?
deregulation of calcium homeostasis, which is related to diseases
What happens if there is an alteration in the mitostatic process of purkinje cells?
mitochondria accumulate in the some and cannot be transported to the periphery, resulting in the accumulation of calcium at the synaptic terminals
What encompasses cerebellar ataxias?
genetic diseases in which the mitochondrial calcium axis is impaired
What characterizes cerebellar ataxias?
neurodegenerative diseases that mostly affect Purkinje neurons
there is cerebellar atrophy and loss of Purkinje cells, which leads to loss of coordination and balance and poor speech and eye movements
*these diseases are not lethal
What is a task an ataxic subject cannot complete?
touching their nose
What is a very rare form of cerebellar ataxia?
autosomal recessive cerebellar ataxia (ARCA)
What genes are mutated in ARCAs?
nuclear genes that are imported into the mitochondria
What is the largest example of ARCAs?
m-AAA-associated ataxias
What is the largest example of ARCAs?
m-AAA-associated ataxias
Where is the m-AAA complex located?
IMM
What kind of complex are m-AAA? Why are they important?
hexamers that exert the protein QC
What 2 types of m-AAA complexes can be found in humans?
oligomeric: composed of AFG3L2 subunits
heterooligomeric: composed of AFG3L2 + paraplegin
What is paraplegin?
protein highly homologous to AFG3L2
What do biallelic AFG3L2 mutations lead to?
severe childhood disease called SPAX5, which is lethal and patients die around 13-14 years of age
How do heterozygous AFG3L2 mutations lead to?
2 diseases depending on the affected domain:
spino-cerebellar ataxias (SCA28): lead to loss of Purkinje cells and are proteolytic domain mutations
optic atrophy 11 (DOA11): leads to loss of retinal ganglion cells and are AAA (ATPase) domain mutations
What do biallelic mutations in SPG7 (gene encoding paraplegin) lead to?
spastic paraparesis and cerebellar ataxia
What do mutations of the m-AAA, in particular AFG3L2 do?
alter the mitochondrial network morphology, which results to be fragmented in the absence of the protein
What leads to Purkinje cell degeneration?
alteration of the mitochondrial network → mitochondrial trafficking is impaired → organelles accumulate in the soma → synapses are devoid of mitochondria → mitochondrial membrane potential is lost → less ATP is produced → calcium is not properly buffered at the synapsis
What could be a possible way to rescue several ARCA forms if calcium is the endpoint of the pathways?
targeting the downstream event (Ca2+ deregulation)
What is a drug being used to target the downstream event to rescue several ARCA forms?
Ceftriaxone (beta-lactam antibiotic), which is also used to cure meningitis and other infections of the CNS since it can pass the blood-brain barrier
What are some benefits and non-severe side effects of Ceftriaxone?
transcription and activity of glutamate transporter are enhanced
What is Ceftriaxone capable of?
potentiate the glutamate internalization of astrocytes, which leads to the reduction of glutamatergic stimulation of purkinje cells and therefore of the calcium influx in them
What is important to remeber about Ceftriaxone?
it is only targeting the downstream events and does not rescue the damaged mitochondria
What is a master regulator of the processing of OPA1?
AFG3L2
What is OPA1 a mediator of?
mitochondrial inner membrane fusion
What 2 proteases are involved in mitochondrial inner membrane fusion? Which one is stress sensitive?
YME1L1 and OMA1
OMA1 is stress sensitive
What does the loss of long OPA1 forms lead to?
impairment of fusion and the fragmentation of the mitochondrial network
What does the mutation or deletion of AFG3L2 enhance?
OPA1 processing and degradation, by over activating OMA1
Why does the mutation or deletion of AFG3L2 enhance OPA1 processing and degradation by overactivating OMA1?
upon cleaving OPA1, OMA1 undergoes a self-cleavage in order to regulate the process since it is dangerous to have OMA1 always active in mitochondria
What else can AFG3L2 regulate?
turnover of proteins encoded by mitochondrial DNA
What does the accumulation of mito-encoded peptides (ND1) upon the loss of AFG3L2 create?
proteotoxic stress in the mitochondria, which leads to OMA1 over-activation and OPA1 over-processing, which results in mitochondrial fragmentation
Is it possible to counteract the accumulation of ND1 and rescue the phenotype?
potentially by treating with chloramphenicol, which is an antibiotic that interferes with bacterial ribosomes and mitochondrial robisomes
*the results show that the presence of the antibiotic completely restored the OPA1 long forms and the mitochondrial fusion was also restored
What causes the mitochondrial proteotoxic srtress?
accumulation of the mitochondrial-encoded polypeptides
What is an “integrated stress response”?
caused by the accumulation of the mitochondrial-encoded polypeptides (aka mitochondrial proteotoxic stress)
What does the stress in the mitochondria trigger?
a cytosolic response whose aim is to shut down translation and allow cell recovery
What happens when the mitochondria release a stress signal?
activation of a kinase that phosphorylates eIF2𝛼, which is a translation initiation factor
- the phosphorylation shuts down translation in order to reduce the load of proteins that the cell has to manage to recover
What does eIF2𝛼 promote the translation of?
ATF4, which is a transcription factor that allows the transcription of may cytoprotective genes that help cell recovery
