Lecture #5 Flashcards
can exosomes engulf a whole mitochondria?
no - only in fragments
what are some components of the mitochondria that are highly immunogenic?
- the inner membrane lipid cardiolipin
- the cytochrome C
- mitochondrial DNA
what are all three immunogenic components of mitochondria recognized as?
DAMPS - damage associated molecular patterns
what is transmitophagy?
the process in which neurons deliver damaged mitochondria to astrocytes
when does transmitophagy occur?
when the load is excessive in the neurons and they can’t manage to degrade all the damaged mitochondria
what happens when the CNS releases DAMPs?
these molecules are taken up by microglial cells → macrophages of the CNS
what is the effect of microglial cells releasing DAMPs?
further neuron damage - associated with chronic inflammation
when might microglial cells release entire mitochondria?
if they are damaged
describe neurons:
polarized cells with extreme morphology
what characteristic of mitochondria is crucial for their function?
their need for rejuvenation (proper turnover)
what is mitostasis?
a specialized form of homeostasis by which the mitochondria number and quantity are maintained over time
what are the two levels of mitochondrial control that encompass mitostasis?
molecular (control of the proteome) and organellar (control of mitochondrial dynamics)
what are the player of fusion and fission that are dynamin-like GTPases?
- DRP1
- MFN1 & 2
- OPA1
what are the two types of transport in neurons?
long range and short range
what type of transport occurs in the axons?
long range transport
what is long range transport based on?
based on microtubules and molecular motors that can mediate the anterograde transport and retrograde transport of mit
how does long range transport compare to short range transport?
LRT is much faster
what is short range transport based on?
based on microfilaments of actin or neurofilaments → mit are bound on the cytoskeleton by different motor proteins
why do we need fast transport in axons?
we have to deliver the mit far to synapses
how is speed affected when mitochondria shift from the microtubules to the actin or neurofilament?
the movement becomes slow because the mit need to stay there for a long time in order to provide the ATP to buffer the calcium locally
what is the best form for mitochondria to be transported?
it needs to be in the intermediate state → not completely fragmented but with short tubules
what is the effect on transportation if there are issues with mit fission?
if we lose fission the mitochondria are more elongated, and they can generate giant balloons difficult to transport
what happens when mitochondria are highly fragmented?
there is not transport
what are two crucial events important to neurons played by mitochondria?
ATP production and Ca2+ buffering
where does 90% of the ATP come from?
provided by mitochondria
why must mitochondria be turned over?
sometimes the journey of the mit from the soma to the synapses excess the life of the mitochondrial proteins → there must be local synthesis and local degradation of mit proteins
what local mechanism is involved in mitochondrial metabolism?
the synthesis of proteins → ribosomes are located in the axons and is driven by the needs of the mitochondria
what is a challenge that neurons have to face in regards to energy requirements?
local demand matching → the demand for energy and calcium buffering depends on the regions, the synapses will need the highest level of both
what is axonal transport?
the transport of mitochondria from the cell body to the periphery on axon microtubules
describe the directionality of mitochondrial transport on axons:
bidirectional → mitochondria move both retrogradely and anterogradely - they make long or short pauses and can change the direction of their journey
what percentage of the mitochondria are stationary in the CNS?
almsot 70% → we need them in specific locations to exert their functional roles
what is the average velocity of mitochondria?
.3-2 µm/sec
what is a Kymograph analysis and what does it show us?
overview of mitochondrial transport → vertical bars represent stationary mitochondria and oblique lines indicate retrograde or anterograde mit
what is seen regarding mitochondrial movement in primary mitochondrial disease?
difficulties in transport with a large accumulation in the stroma
what is the “dying-back pathology” in neurons?
neurons don’t remain healthy → since the mitochondria are not transported to the terminals, the axon starts degenerating from the back to the stroma causing it to die
what mitochondrial motor is required for anterograde movement?
kinesins
what mitochondrial motor is required for retrograde movement?
dyneins
what is the motor for the actin cytoskeleton?
myosin V (A)
describe the moving of the mitochondrial potential?
it is equal, regardless of whether it is moving retrogradely or anterogradely → whether they are damaged or healthy they are damaged or healthy in BOTH directions
why is it more difficult to study mitochondrial transport in dendrites?
they are smaller and in the proximal dendrites, where the microtubules are very short and have mixed polarity
what is mitochondrial movement mediated by in dendritic spines?
mediate by microfilaments
in kinesins, dyneisns, and myosins, what is the head domain important for?
interactions with microtubules → head domain walks along the microtubules
in kinesins, dyneisns, and myosins, what is the tail domain important for?
interacts with the cargo (mitochondria)
how many types of kinesins are there in mammals?
3
what is the most important kinesin in neurons?
KIF5A
what is the function of dyeins?
mediate retrograde movement towards the minus end of microtubules
what do dyneins allow for?
the long distance transport of mitochondria and other organelles on microtubules though mechanisms that require ATP hydrolysis
what is the function of Myosin V?
motor protein and a possible transporter of mitochondria on actin filaments, which are enriched in synaptic terminals and in distal dendrites and are responsible for mitochondrial docking
how are mitochondria bound to microtubules?
though kinesins
what happens once mitochondria reach the terminals and detach from the microtubule?
they attach to the microfilaments via Myosin V
how do mitochondria attach to the motor proteins?
through adaptor proteins on the outer membrane
besides attachment, what is another function of adaptor proteins?
also give specificity of transport
what specifically on the outer membrane gives specificity of transport?
Miro1
describe Miro1:
calcium binding protein with two EF and GTP domains
what does Miro1 interact with?
Milton → form the Miro/Milton complex which interacts with kinesins and dyneins
there are at least four levels of regulation for mitochondrial transport, what is the the first and most important?
mediated by calcium levels
how is mitochondrial transport mediated by calcium levels?
if there is a very high calcium increase in the neurons, the 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, and trapped by the Miro-Milton complex.
what is calcium used to trigger in mitochondria movement?
calcium is needed to stop the mitochondria in the site where they have to exert calcium buffering
what does the regulation of mitochondrial transport depend on?
cellular needs
what else besides calcium can make mitochondria stop and anchor it to the microtubule?
Syntaphilin → anchors all stationary mitochondria to the microtubules
what happens when mitochondria are depolarized during transport?
activates the PINK1-Parkin pathway
what other protein is degraded by PINK1-Parkin pathway upon ubiquitination?
Miro1
mitofusins are part of the omm as well, so what happens when they get ubiquinated?
in the case of depolarization and the loss of transport, there is a loss of fusion as well
where does mitophagy occur?
in the distal regions of the axon
where are Purkinje neurons located?
in the cerebellar cortex
what is the function of Purkinje neurons?
they are GABAergic neurons with inhibitory function, they make synapses with the deep cerebellar nuclei, which are in contact with the spinal neurons
what do Purkinje neurons exert control on?
fine control of movement and coordination
what is strictly related to Purkinje neuron health and functionality?
mitostasis - strictly correlated to the calcium homeostasis pathway → impairment of mitostasis results in the degradation of calcium homeostasis
why is it hypothesized that Purkinje neurons have such a high reliance on mitochondria?
they have a very high oxidative metabolism → they have a considerable demand of high amounts of ATP levels and Ca2+ buffering
what is different about Purkinje neurons compared to normal neurons?
post-synaptically they mainly receive excitatory stimulation (glutamatergic), so they experience very large amounts of calcium fluxes, higher compared to other neurons → they have spontaneous firing properties
what does any alteration of mitochondrial transport lead to in Purkinje neurons?
causes mitochondria to accumulate in the soma of the Purkinje cells and not be transported to the periphery
what does an accumulation of mitochondria in Purkinje cells cause?
a pathological accumulation of calcium in the synaptic terminals
what is another possible cause of the deregulation of calcium homeostasis besides the accumulation of mitochondria?
low ATP levels since ATP fuels the activity of the calcium-clearance system (SERCA)
what are diseases involving cerebellar ataxias characterized by?
cerebellar atrophy and loss of Purkinje cells, with consequent loss of coordination and balance, difficult movements, and, in the final stages, the patients also have poor coordination of hands, speech and eye movements
describe cerebellar ataxias diseases in terms of genetics:
have a high genetic heterogeneity and are autosomal dominant, autosomal recessive, and X-linked forms
what are ARCAs?
autosomal recessive cerebellar ataxias
what is the biggest example for ARCAs?
m-AAA associated ataxias
what is m-AAA?
a complex located in the imm that is important to exert the protein quality control in the organelle
what are the two types of m-AAA?
oligomeric - AFG3L2
hetero-oligomeric - AFG3L2+ paraplegin
what two diseases can heterozygous AFG3L2 mutations lead to?
SCA28 and DOA11
what is SCA28?
heterozygous AFG3L2 mutation in the proteolytic domain related to spino-cerebellar ataxias (SCA28) that leads to the loss of Purkinje cells
what is DOA11?
AAA (ATPase) domain mutation that results in optic atrophy 11, which is characterized by the loss of retinal ganglion cells
why do the two mutations of heterozygous AFG3L2 affect different cell types?
Purkinje and retinal ganglion cells are similar in terms of morphology non energy requirements
what do biallelic mutations in SPG7 (gene encoding paraplegin) lead to?
lead to spastic paraparesis and cerebellar ataxias
what do mutations of the m-AAA, in particular AFG3L2 lead to?
they are the mitochondrial network morphology, which causes it to be fragmented in the absence of this protein
when mitochondria are damaged and lose their membrane potential, they produce less ATP and are unable to do what?
buffer calcium at the synapsis → leads to Purkinje cell degeneration
the pathogenesis of these diseases is characterized by a cascade of events in which what happens?
the mitochondrial netowork is impaired which leads to impairment of mitochondrial trafficking and the accumulation of mitochondria, leaving the synapsis completely empty
what is a beta-lactam antibiotic that can pass through the blood brain barrier (also used to treat other issues in the CNS and meningitis)?
Ceftriaxone
what does this drug allow for?
it is possible to potentiate the glutamate internalization of astrocytes, that leads to the reduction glutamatergic stimulation of Purkinje cells, and therefor the calcium flux in them
what is important to remember about Ceftriaxone?
it targets only downstream events and does not rescue damaged mitochondria
what is AFG3L2 a master regulator of?
OPA1 → OMA1=stress sensitive protease
what does the loss of OPA1 lead to?
the impairment of fusion and the fragmentation of the mitochondrial network
what does a mutation in AFG3L2 do in regards to OPA1?
enhances OPA1 processing and degradation by over-activating OMA1
how does a mutation in AFG3L2 enhance OPA1 processing by over-activating OMA1?
upon cleaving OPA1, OMA1 undergoes a self-cleavage, in order to regulate the whole process, since it is very dangerous to have OMA1 always active in mitochondria.
It was, also, demonstrated that AFG3L2 regulates the turnover of the proteins encoded by mitochondrial DNA
the very first step upon the loss of AFG3L2 is the accumulation of mito-encoded polypeptides - what does the accumulation of these products cause?
STRESS= the over activation of OMA1 therefore leading to over processing of OPA1 resulting in fragmentation
what is mitochondrial proteotoxic stress?
caused by the accumulation of the mitochondrial-encoded polypeptides, generating an “integrated stress response”
what does the integrated stress response trigger in mitochondria an what is the goal?
triggers a cytosolic response whose final aim is to shut down translation and to allow for cell recovery
what is the final outcome of the stress signal released by mitochondria into the cytosol?
the activation of a kinase that phosphorylates eIF2⍺ (translation initiation factor) → shuts down translation in order to reduce the load of proteins the cell has to manage so it can recover
besides shutting down translation so the cell can recover, what else does eIF2⍺ promote the translation of?
ATF4 → transcription factor that allows the transcription of many cytoprotective genes to help cell recovery
Autosomal dominant cerebella ataxia’s are related to what subclass of the disease?
SCAs - 48 loci
Autosomal recessive hereditary cerebella ataxia’s coincide with what subclass of the disease?
ARCAs - 98 loci
