role of the cytoskeleton

Question 1

what is the cytoskeleton?

Answer 1

dynamic scaffold inside all eukaryotic cells that is responsible for cell shape and motility as well as transport and organization

Question 2

how does the cytoskeleton assist neuronal function?

Answer 2

1. cytoskeletal function
2. protein localisation
3. intracellular transport
4. vesicle fusion and recycling

Question 3

what are cytoskeletal networks made up of?

Answer 3

actin
microtubular networks
intermediate filaments

Question 4

where is actin found?

Answer 4

around the plasma membrane to provide structural strength

Question 5

what mechanism are microtubules important for?

Answer 5

transportation

Question 6

what is the importance of intermediate filamemts?

Answer 6

give cell overall shape and strength

Question 7

what are microfilaments made of?

Answer 7

actin

Question 8

what is the size of actin?

Answer 8

~5nM in diameter, thin fibres found throughout cells

Question 9

what are the two forms of actin?

Answer 9

monomeric G-actin and polymeric F-actin (filamentous)

Question 10

G-actin description

Answer 10

ATP-bound (G-actin has a binding site for ATP)
can use ATP to make F-actin
ATP-bound G-actin has a higher affinity for the filament and polymerizes more readily at the plus end

Question 11

F-actin description

Answer 11

F-actin is a double-stranded helix, where each strand is composed of G-actin monomers
has polarity, with a plus (+) end (fast-growing, barbed end) and a minus (−) end (slow-growing, pointed end)

Question 12

action of actin

Answer 12

found extensively just below the plasma membrane and
forms thin filament in muscle fibres – binds myosin molecules
during contraction

Question 13

what is the size of microtubules?

Answer 13

~20nm diameter thick fibres

Question 14

what are microtubules formed by?

Answer 14

polymerisation of alpha and beta tubulin which takes the form of a spiral walled tube, forming networks throughout the cytoplasm

Question 15

what are microtubules crucial for?

Answer 15

transport of vesicles and organelles through out the cell
forms the mitotic spindle

Question 16

what is the energy source of microtubules?

Answer 16

GDP and GTP

Question 17

what is the size of the intermediate filaments?

Answer 17

~10nm medium fibres

Question 18

formation of intermediate filaments

Answer 18

monomers form dimers by sense-sense double helix
tetramers are formed by two dimers wrapping round each other to form sense-antisense double helix
tetramers coalesce and wind round each other to form the final filaments (c.f rope) which creates a very stable structure for mechanical strength to cell structure

Question 19

neurofilaments (neuronal intermediate filamemts)

Answer 19

three protein subunits: NF-L (light), NF-M and NF-H
NF-M and NF-H can be heavily phosphorylated and are found mostly in axons

Question 20

importance of NF-H and NF-M in neurofilaments

Answer 20

provide mechanical strength to axons – particularly highly expressed in motor neurons which have very long axons

Question 21

what does phosphorylation in neurofilaments control?

Answer 21

phosphorylation controls axonal diameter – high
levels of phosphorylation = large diameter axon

Question 22

actin and microtubule distribution in axons

Answer 22

microtubules are found throughout the axon
actin is found in the growth cones

Question 23

actin and microtubule distribution in dendrites

Answer 23

microtubules are found throughout the dendrites
actin is found in the dendritic spines

Question 24

actin polymerisation

Answer 24

at the end of the growth cone/spines, actin is perpendicular
as it grows it extends forward and pushes the membrane forward
monomers at the end are recycled
this is how actin drives growth

Question 25

cytoskeletal organisation of dendritic spines

Answer 25

actin filaments are found in dendritic spines but not microtubules spine head (actin parallel to membrane), spine shaft and dendritic shaft (contains mictotubules)

Question 26

microtubules act as a...

Answer 26

motorway network in both dedrites and axons

Question 27

MAPS (microtubule associated proteins)

Answer 27

polymeric assembly of microtubules is stabilised by MAPS axon MAP= tau dendrite MAP= MAP2

Question 28

protein localisation due to the cytoskeleton

Answer 28

dendrite= postsynaptic receptors and ion channels axon hillock= sodium channels NOR= sodium and potassium channels pre-synaptic bouton= calcium channels and and autoreceptors

Question 29

axonal localisation- actin, spectrin and ankyrin G

Answer 29

spectrin= dimer formed by two monomers in a double helix, protein binds ankyrin G (anchor protein, binds to cytoskeleton and ion channels) 1. leads to clustering of channels 2. generates high concentrations of Na+ channels at the axon hillock 3. localises Na+ and (some) K+ channels at nodes

Question 30

dendritic actin organisation

Answer 30

1. actin is organised in short longitudinal strands 2. spectrin is likely to be much less dense 3. much lower degree of ion channel clustering 4. ‘hot spots’, esp near branch points however there are regions that are similar to axons= branching points

Question 31

post synaptic density anchoring

Answer 31

contains LOTS of proteins some of the most important are the PDZdomain proteins e.g. GRIP, PICK= crucial for the localization and stabilization of signaling proteins MAGUKs=membrane associated GUanylate kinase e.g.PSD95= scaffold proteins that organize and stabilize protein complexes PSD is ultimately anchored to the actin cytoskeleton anchor proteins at the post synaptic membrane

Question 32

weiss and hiscoe (1948)

Answer 32

1st evidence for movement in axons * ligated peripheral nerves * noticed accumulation of material on the proximal (cell body) side of the constriction, and depletion on the distal (axon terminal) side * showed that materials are actively transported from the cell body down the axon via anterograde transport

Question 33

axoplasmic flow

Answer 33

refers to the movement of cytoplasm within the axon of a neuron slow rate (1 - 2 mm/day) structural proteins (e.g. tubulin, neurofilaments)

Question 34

what is axonal transport (bidirectional) driven by?

Answer 34

fast axonal transport (400mm/day) supported by ATP hydrolysis

Question 35

anterograde transport

Answer 35

* begins in cell body * cargo transported out to the axon terminus / processes * powered by a kinesin

Question 36

retrograde transport

Answer 36

* begins at axon terminus * cargo transported back through the axon to cell body * powered by MAP-1C (microtubule associated ATPase) – a dynein

Question 37

kinesin description

Answer 37

heterotetramer comprising of 2 heavy and 2 light chains (double helix) heavy chains contain a globular head (ATPase domain) globular heads act as motors when attached to microtubules travel towards the + end have vesicle attached at end which carries receptors

Question 38

what are microfilaments?

Answer 38

made of actin protein subunits that form long, thin, flexible helical structures mechanical support, facilitate cell movement, muscle contraction etc

Question 39

what are intermediate filaments?

Answer 39

made up of fibrous proteins that assemble into rope-like structures and coiled into a helical structure provide structural stability and anchor organelles

Question 40

what are microtubules?

Answer 40

hollow tubes made up of tubulin protein subunits (alpha and beta tubulin dimers) have rigid and have dynamic properties meaning they can grow and shrink provide structural support and intracellular transport

Question 41

dynein description

Answer 41

complex of heavy, intermediate and light chains heavy chains contain a globular head (ATPase domain) globular heads act as motors when attached to microtubules travel towards the - end

Question 42

what are kinesin and dynein?

Answer 42

two types of motor proteins that move along microtubules these proteins convert chemical energy stored in ATP into mechanical work to facilitate intracellular transport