Natalie Gardiner Flashcards
what is the order of movement of info?
synapse - dendrite - soma - axon - synapse
describe the development of neuronal polarity
Plated neurons make lamellipodia.
neurons extend several equal length minor processes.
one process beings to grow rapidly.
remaining processes grow slowly and acquire dendritic characteristics.
neurons fully polarised, synapses begin to form. (day 7-10)
very fast
describe the polarity of axons and dendrites
Axons – all microtubules oriented with Plus ends outwards, and associate with specific binding proteins to stabilise polymers
Dendrites – Mixed orientation of microtubules in dendrites
what is the cytoskeleton made of?
microtubules
microfilaments
intermediate filaments
describe microtubules
tubulin
25nm largest
hollow tubes
GTP nucleotide
describe microfilaments
actin
7nm smallest
helical filaments
ATP nucleotide
describe intermediate filaments
vimentin/neurofilaments
10nm middle
rope like filaments
no bound nucleotide
describe microtubule structure
a and b tubulin are globular proteins.
associate by non covalent bonds to form a ab heterodimer.
heterodimers join to form a protofilament.
13 protofilaments form a mature microtubule in a cylinder.
each monomer has a binding site for 1 molecule of GTP.
other binding sites for proteins/drugs.
has polarity.
- end a tubulin exposed. slow growing.
+ end b tubulin exposed. fast growing, heterodimers add to this end.
what can tubulin subunits do?
enzymes that catalyse GTP hydrolysis to GDP
how many protofilaments form a mature microtubule in a cylinder.
13
what are the forms of tubulin?
T form and D form
most free are T form.
D form has been hydrolysed
T form are recruited to the plus end.
subunits in D form shed from the minus end
what is the GTP tubulin cap?
stable, strong bond.
??
what is tradmilling.
equal number adding and being taken off.
polymer constant length
what is dynamic instability
if GTP hydrolysis is faster than subunit addition.
what is a loss of GTP cap?
everything is hydrolysed, all bonds become weaker.
curvature and shrinking.
what is a “catastophe” depolymerisation.
structure breaks down.
rescued by new GTP tubulin.
what molecules can slow down the hydrolysis to GDP and stabalise the molecules?
XMAP215, EB1 and CLIP-170
what are microtubule associated proteins (MAPs)
can be large/small.
bind to molecule, slow down GDP hydrolysis and stabilise it.
promote MT polymerisation.
also organise and anchor organelles.
what do neurofilaments dictate?
axonal diameter.
space themselves out by mutual charge repulsion of sidearms.
why is axonal diameter important?
faster the conduction rate.
myelinated axons?
where are neurofilaments added on?
along the width of the NF, as well as at the ends.
describe actin
single globular polypeptide
binding site for ATP
assemble head-tail to generate protofilaments.
unrelated to tubulin.
2 parallel protofilaments twist in a right-handed helix – to form a flexible microfilament. organised into linear bundles, particularly concentrated beneath plasma membrane.
Like MTs, actin filaments have polarity and undergo treadmilling and dynamic instability (mediated by ATP hydrolysis rather than GTP).
Actin filaments can associate with accessory proteins – to form stable, large adhesion complexes – linking internal cytoskeleton with extracellular environment.
describe the actin cytoskeleton and the growth cone.
compare to dendritic growth cones.
Actin cytoskeleton is less condensed/more dynamic in axon growth cone which allows microtubules through to drive axon outgrowth.
In contrast, dendritic growth cones have a more rigid cytoskeleton – therefore less elongation
sum the role of the neuronal cytoskeleton.
MTs confers polarity to the neuron.
MTs play a crucial role in maintaining structure and strength and organelle positioning within the cell
NFs play role in tensile strength, axonal calibre and conduction velocity.
MFs also have polarity and play active role in rapid outgrowth, growth cone dynamics and anchoring components
Dynamic remodelling of the neuron by MTs and MFs drives development, regeneration and plasticity
MT form the ‘Tracks’ for specialised delivery of proteins – ‘axonal transport’
describe the physical lesion of a MT for axonal transport
tie a knot.
see which side proteins accumulate, can tell where they are coming from.
axonal transport.
what need to know?? end of lecture 7
where are proteins packaged?
golgi apparatus.
what is Anterograde axonal transport?
Away from the cell soma
what is retrograde axonal transport?
towards the cell soma
describe commonalities in dendrites
branched.
characteristic taper, get narrower as they get to the distal ends.
emerge from soma.
cytoplasm continous with soma.
what is the earliest point of dendritic spine maturation?
filopodium
how does the shape change of dendritic spines?
filopodium - thin - stubby - mushroom - cup
summarise dendrites
Dendrites come in a diverse range of shapes and sizes
Specialised for receipt of information
Play significant role in signal processing – shape, distribution of channels & synapses
Receive the vast majority of a cell’s synaptic inputs
Key roles in learning, memory and behaviour
what are the domains of the axon?
axon hillock
axon shaft
axon terminal
also nodes of ranvier
describe the axon hillock
A 1-10um region which demarcates the somatodendritic-axonal boundary
tapered structure
Physical diffusion barrier
High concentration of ankyrin G and voltage gated sodium channels
Plays a critical role in integration
Rarely innervated
very few inputs - Axons that do synapse onto hillock are usually from inhibitory neurons (GABA)
(Powerful way to shut down all inputs cell receives in one swoop)
what is selective endocytosis/elimination retention?
Elimination/retention - channels uniformly inserted in the neuronal membrane, then removed from dendrites, soma and regions of the distal axon by endocytosis
what are the methods of compartmentalisation of NaV channels?
direct insertion/selective endocytosis.
both are used
describe ankyrin G
plays a rolein the organisation of the AIS e.g.
localisation and immobilization of Sodium Channels
AIS is a diffusion trap for these proteins.
forms an association between Na channel and celedesian ????
important for integration.
describe the axon shaft
Axon emerges from soma
Untapered cylindrical structure
Axons typically 1um wide, and can extend metres in length (largest cells in body (volume and surface area)
Collateral Branches can emerge at 90o
Can be myelinated (Schwann cells in PNS; oligodendrocytes in CNS) or unmyelinated
Rarely innervated
myelination of PNS/CNS?
Schwann cells in PNS; oligodendrocytes in CNS
1 oligodendrocytes can myelinate several axons/internodes
schwann cells 1:1 ratio
describe the axon terminal
1um->10um wide – terminals are species & target dependent
can be innervated:
Contains synaptic vesicles, ER, polysomes, Mitochondria
High density of calcium channels
Active Zone where transmitter released
axon growth uses guidance cues - attractants/repellents
comparisons of axons v dendrites
on lecture 8 table DRAW OUT
how are microtubules distributed along the axon?
Not one microtubule stretches the entire length of the axon (instead there are overlapping parallel microtubules )
what are the rates of axonal transport?
Fast Axonal Transport: Movement of membrane-bound proteins at ~ 200-400mm/day
Slow Axonal Transport: Movement of non-membrane bound proteins ~ 0.1–20mm/day
what are the directions of axonal transport?
Anterograde (from cell body towards axon/synapse)
Retrograde (from axon/synapse towards cell body)
describe fast anterograde transport.
transports membranous organelles: ie vesicles/mitochondria
mediated by microtubule-binding protieins (kinesin/kinesin superfamily of proteins KIFs)
describe KIFs
KIFs possess a conserved globular motor domain which includes an ATP-binding sequence and a microtubule binding sequence.
The other regions - filamentous ‘stalk’ region and globular ‘tail’ region are more variable. not always present
Most KIFs have cargo binding site in tail region for membrane bound vesicles/organelle.
describe N and M kinesins
N:
NH2 terminal moves to plus end
M: middle domain moves to plus end
describe C kinesins
COOH terminal moves to minus end
describe the movement/binding of kinesin
2 identical motor heads which both have a catalytic core.
both have ADP.
kinesin head binds to microtubule, causing ADP to be released.
ATP enters the empty nucleotide binding site.
triggers the neck linker to zipper onto the catalytic core, throwing the second head forward, binds to microtubule site.
hydrolyses ATP and releases phosphate.
neck linker unzippers from trailing head, leading head exchanges ADP for ATP and zippers its neck linker on.
cycle repeats along the microtubule.
describe microfilament based transport
myosin driven.
Microfilament-rich regions, utilise myosin motors to generate movement along microfilament tracks.
Central force-generating element similar to kinesin - including site of ATP binding and subsequent hydrolysis triggering allosteric conformational change.
But, much slower than kinesin on MTs
describe fast retrograde transport
Performed by the specialized protein motor dynein operating along microtubule tracks
Returns endosomes (endocytic vesicles), mitochondria etc to soma for degradation in lysosomes and recycling.
Transports endocytosed neurotrophins/cytokines to soma
Transports certain toxins (tetanus) and viruses (herpes, varicella, rabies, polio, HIV) to soma
describe dynein
Dynein is a huge (>1000kda) MINUS end directed microtubule binding motor protein
ATPase activity and binds microtubule
Requires association with a second large protein
complex called dynactin – weakly binds MTs
Dynein-associated accessory proteins impart
cargo specificity
describe slow axonal transport
Transports:
Cytosolic and cytoskeletal proteins (e.g. neurofilaments).
Enzymes for small neurotransmitter synthesis transported from soma to axon terminal.
not sure how it takes place. few hypothesis.
describe the hypothesis of slow transport
structural hypothesis.
Original studies using radioisotopic pulse labelling found NFs moved at an average rate of 0.25−3 mm per day. ‘Structural hypothesis (Lasek) – suggested diffusion
BUT - Advances in live-cell imaging finds actual rate
of NF movement can be much faster
(0.38 µm/s).
BUT: movements are interrupted by prolonged
pauses (85−99% of their journey along the axon
is spent stationary. ‘Stop and Go’ hypothesis.
Likely multiple mechanisms contribute to slow
transport (active & diffusion)
stop and go hypothesis?
can move very fast but also stationary for long periods of time
what are the two mechanism of motor based trafficking?
local protein synthesis
mRNA shipped to site and translated there.
distal protein synthesis
mRNA translated at soma and shipped to site.
what mRNAs are trafficked?
ones that are needed for giving a rapid response.
synaptic plasticity.
what are the two mechanisms proposed for distal protein synthesis. getting the proteins to the right place
direct targeting (smart motors):
- Recognise proteins as either axonal or dendritic.
- Only transport proteins to correct locations.
indirect targeting (stabilization):
- Transport of proteins everywhere, inserted into no specific locations.
THEN Recognition occurs at plasma membrane
- Protein regarded as stable or unstable depending on location.
- Appropriate components retained, and inappropriate ones internalised.
Both occur
summarise
Neurons rapidly polarize during development
mRNA and protein components delivered and maintained selectively to axons and dendrites
Local mRNA translation – affords rapid responses
Environmental influences cause local changes
e.g. electrical activity, synapse formation
