Lecture 10: Cellular Structure of the Brain 2

Question 1

what is special about neurons?

Answer 1

shape and size - unique as it is dependent on where input is from, location, and where their axons have to go

Question 2

Shape of the neuron

Answer 2

determined by connections - input/output

Question 3

how is neuron shape maintained?

Answer 3

proteins

Question 4

what is different between the dendrite and axon?

Answer 4

membrane proteins

Question 5

What does the cytoskeleton help to do

Answer 5

maintain and make shapes and therefore maintaining the function of the neurons

Question 6

what dimers bind together to form microtubules?

Answer 6

alpha and beta tubulin

Question 7

add tubulin dimers to which end of the microtubule?

Answer 7

add tubulin dimers at the positive end to elongate axon (grows towards the end of the axon)

Question 8

MAP-2

Answer 8

dendrite specific also the soma

Question 9

Tau

Answer 9

dendrite and axon

enriched in distal axon

Question 10

Microtubule uniform orientation

Answer 10

positive ends towards axonal end

microtubules are highly organised in the axons and dendrites, microtubules bound by tau in the axons which stability the structure and they also have uniform extension

Question 11

microtubule mixed orientation

Answer 11

positives at both ends therefore both ends can extend which allows for extending and retracting

MAP2 allows for stabilising of structures in dendrites forming parallel networks

Question 12

loss of MAPs

Answer 12

tangled microtubules which disrupts structure which disrupts cell which disrupts function

Question 13

Material is transported down the axon via

Answer 13

microtubules (highways of the cell)

Question 14

How was axonal transport found out about?

Answer 14

Axonal transport – 1930’s - squid neuron
Ligate axon, vesicles accumulate on soma side
Inject label into soma and watch it move down axon - moved down faster than rate of diffusion therefore realised something must actually be transporting them
The first electron microscope made in 1931.

Question 15

Fast axonal transport

Answer 15

bidirectional (250-400mm/day)

Question 16

Slow axonal tranport

Answer 16

anterograde (1mm/day) - transporting more structure molecules therefore do not need as fast flow as fast axonal transport

Question 17

anterograde axonal transport carries

Answer 17

mitochondria, vesicles, membrane lipids

Question 18

Retrograde axonal transport

Answer 18

used materials

purpose = recycle and repair, materials that need to be recycled e.g. degraded materials or nonfunctioning mitochondria

Question 19

_______ proteins and axonal transport

Answer 19

uses motor proteins
kinesin - antero
dynein - retro

Question 20

Motor protein for anterograde axonal transport

Answer 20

kinesin

Question 21

motor protein for retrograde axonal transport

Answer 21

dynein

Question 22

Kinesins vs dynein

Answer 22

Kinesins govern the majority of anterograde transport and dynein is responsible for retrograde transport, shuttling proteins back toward the cell body for recycling

Question 23

motor protein consists of

Answer 23

motor domain, tail domain

Question 24

Motor protein - motor domain

Answer 24

contains ATPase (uses ATP) 
conserved across species

Question 25

Motor protein - tail domain

Answer 25

specifies function of motor molecule diverse within/across species attaches on to the organelle/vesicles that is going to be transported

Question 26

Cargo and motor protein tail

Answer 26

Cargo have distinct mechanisms to select and dock to the correct motor protein tail when you put something into a vesicle you call it a cargo the vesicle does not have a simple plasma membrane, lots of proteins associated which act as signposts/label which attach onto the motor protein which can take that organelle down to where it is required

Question 27

Neurofilament overall function

Answer 27

maintain the strength and structure of neurons

Question 28

diameter of neurofilaments

Answer 28

10nm

Question 29

Neurofilament features

Answer 29

``` predominant cytoskeletal component ́ = intermediate filaments huge mechanical strength ́most stable of cytoskeleton ́important in structural framework - very dense and provide the structure, they are very stable and very strong ``` neurofilaments in the axon have associated proteins - extensive cross linking gives high tensile strength

Question 30

Neurofilaments are ...

Answer 30

cargos of axonal transport (move along an axon as cargo on a microtubule)

Question 31

Microfilaments made up of

Answer 31

actin - filamentous (F) and monomeric (G)

Question 32

Diameter of microfilaments

Answer 32

3-5nm

Question 33

features of microfilaments

Answer 33

dynamic = continual remodelling of actin filaments | polar - positive (barbed) and negative (pointed) ends

Question 34

F-actin

Answer 34

filamentous | 2 stranded helical filament (weak covalent bonds)

Question 35

G-actin

Answer 35

monomeric

Question 36

Actin tread milling summary

Answer 36

In a microfilament get a net flow of newly acquired G-actin through the filament = known as actin treadmilling: -> a dynamic turnover of actin filaments while filament length is maintained Or increase / decrease length or alter the arrangement good for cell motility and keeping its shape

Question 37

actin dynamics - elongation

Answer 37

add faster at positive end but also adds at negative end just not as much i.e. 3 to 1 for example

Question 38

actin dynamics - steady state

Answer 38

(rapid) polymerisation at the G-actin+ATP site and depolymerisation at the ADP actin site steady state - the concentration of G actin in the cytoplasm reaches a plateu meaning a steady state level of actin dynamics, instead of actin extending on both sides it actually stays the same length (polymerises at one end and depolymerises at the other end) and it uses ATP to do this and we call this stead state level and the change of actin dynamics treadmilling

Question 39

Microfilaments function

Answer 39

Actin –filamentous (F) ́ Mature neuron ́Stabilization – key role (keeping proteins in place) ́many actin binding proteins (a-actinin) ́inner plasma membrane proteins crosslink to actin ́anchors molecules, vesicles (very organised) (and receptors as well) ́in spine head - no MTs (microtubules therefore replaces their function) -> shape ́enriched at synapse, (pre and post) – shape, size and holding proteins ́ Developing neuron / plasticity ́Neurite formation, extension, branching, development of spines and synapses The functional roles for microfilaments involve cell membrane motility, endo- and exocytosis, secretion and vesicle transfer.

Question 40

Actin binding proteins

Answer 40

``` Crosslinking proteins (actin-binding proteins) can arrange F-actin into distinct networks, such as actin bundles, mesh-like and gels. e.g. α-actinin, filamin ``` Actin is important: Mutation in Filamin -periventricular heterotopia, neurons do not migrate properly during the early development of the fetal brain (not having fully functioning actin means that the neurons work differently)

Question 41

Cytoskeletal organisation of dendritic spines

Answer 41

actin is important in dendritic spines actin is also a part of our axonal transport because microtubules cannot get into the spines (too big) so actin replaces its role in the spines - initially on microtubule and then gets to spine so vesicle etc hops off the microtubule using a different motor protein and then actually be transported by the actin protein network into the spine, also actin has a tree like structure which means that it is important for the shape of the spine, which can extend, also important in recycling

Question 42

Actin in the presynaptic terminal

Answer 42

Enriched at presynaptic terminal ́Regulates vesicle pool ́Small groups linked ́Groups attached to plasma membrane ́At periactive zone - ? Involved in vesicle recycling actin important in both pre and post synaptic terminals

Question 43

Pools at the presyanaptic terminal

Answer 43

2 pools releasable pool readily releasable pool

Question 44

Actin in the postsynaptic terminal

Answer 44

submembraneous actin network interlinks scaffolding proteins - organises PSD (post synaptic density) actin filaments regulate surface-receptor diffusion and the eco-endocytic trafficking of receptors to the surface actin - spine shape- transport

Question 45

Can axonal transport go wrong?

Answer 45

‘... experimental studies suggest that the degeneration of terminals and axons precedes the demise of neurons which finally results in the clinical symptoms...” once recognised clinically, the damage to the axonal transport system and other features of neurons has already been done

Question 46

Failure of axonal transport seen in

Answer 46

``` Defects in axonal transport and/or in the cytoskeleton are often seen in the CNS and peripheral neuropathies: Seen in ́ Alzheimer’s disease Motor neuron disease Parkinson’s disease Acquired peripheral neuropathies Diabetic neuropathies Metabolic syndromes Auto-immune diseases Alcohol abuse Anti cancer therapies ``` Symptoms of disease = late We need to know what is happening early -to develop a therapy

Question 47

Diabetic neuropathy features

Answer 47

One putative mechanism of axonal damage in diabetes is impairments to axonal transport. Several alterations can contribute to the disruption of axonal transport: • alterations in the cytoskeleton • molecular motor proteins • cargo proteins • mitochondrial transport and other transport • and possibly, microglia-driven neuroinflammation

Question 48

Summary of high uncontrolled blood glucose in body

Answer 48

high blood glucose in body that is uncontrolled - diabetic neuropathy - affects nerves in legs and feet

Question 49

Diabetes and cytoskeleton

Answer 49

impaired cytoskeleton assembly decrease in axon calibre motor proteins fail to bind microtubules (therefore not as much transport) Impaired transport of synaptic vesicle precursors and mitochondria (?) Neuroinflammation (?) Microglia cells - pro inflammatory molecules (TNF) - affect anterograde transport (microglia become activated therefore proinflammation etc) Less neurofilaments therefore less structure

Question 50

Hyperglycaemia in diabetes (PNS)

Answer 50

Excess glucose molecules allow addition of sugar molecules to protein, e.g. to tubulin and alters its function (seems to be a peripheral axonal tubulin effect, not central.)

Question 51

Microtubules in diabetes

Answer 51

decrease tubulin mRNA increase in tubulin glycation increase in tau phosphorylation increase in tau cleavage

Question 52

Microfilaments in diabetes

Answer 52

increase in actin glycation

Question 53

Neurofilaments in diabetes

Answer 53

decrease in NF-L and NF-H mRNA increase in NF phosphorylation Loss of axonal NFs

Question 54

Overall diabetes effects

Answer 54

decrease axon calibre decrease speed conduction decrease axonal tranport decrease nerve regeneration

Question 55

alzeimers affects the

Answer 55

CNS

Question 56

The changing brain of alzheimers disease

Answer 56

AD brain changes start decades before symptoms show (preclinical AD) cognitive problems, brain dies over time (shrinks)

Question 57

Causes of Alzheimers disease

Answer 57

``` Age and gender genetic factors infections environmental factors others - obesity, diabetes lifestyle cardiovascular disease head injuries ``` central to it all is inflammation