Tissue Plasticity

Question 1

In which human systems is plasticity most evident?

Answer 1

CNS
Muscle (volume, fibre type)

Question 2

What is the connectome?

Answer 2

The pattern of neural connections in the brain and how it changes throughout life
Large number of trial connections formed and pruned during the critical period

Question 3

Synaptic pruning in Muscle fibres- what is involved?

Answer 3

Early developmental expression changes in Ephrin A3/8 to target fast/slow neurons
Fibre terminals swell, lifting off the fibre and cause neuron retraction, leaving only one neuron innervating each fibre

Question 4

Visual acuity and Pruning- what happens?

Hubel 1977

Answer 4

Refined early in development during a critical period lasting 3-8 months
Separation of overlapping ocular dominance columns

Covering an eye (monocular deprivation) widens the contralateral OD column, but shrinks ipsilaterally
Closure of an eye leads to a competitive disadvantage in column retraction/pruning
Differences in Layer 5 (superior colliculus/spinal cord connection pruning)

Question 5

What 4 R’s occur as part of plasticity in adults

Answer 5

Regeneration (new neuron/synapse creation)
Rewiring
Reconnection
Reweighting (changing thresholds for synapse formation)

Question 6

What 4 things make up PNNs?

Answer 6

Chondroitin Sulphate Proteoglycans (CSPGs)
Link Proteins
Tenascin R
Hyaluronan

Question 7

What might PNNs do physiologically?

Answer 7

Neuroprotective role- reducing oxidative stress due to GABAergic neuronal excitability

PNN formation may be Na+ channel dependent, as it is not affected by disruption of glutamatergic system transgenically

Question 8

PNNs and their relation to Visual Acuity

Pizzorusso 2002

Answer 8

PNNs appear at the end of development of visual acuity

- CSPGs are expressed throughout the critical period but are diffusely distributed on immunohistochemistry (IHC)

Question 9

What gene might Pruning be dependent upon?

Answer 9

Otx1

• Transcription factor found to translocate to the nucleus at specific developmental stages
• Otx1 mutant mice are unable to refine/ prune cortical progenitors

Question 10

Models of synaptic pruning

Answer 10

Sequential (formation then elimination)
Concurrent

Pruning has a net turnover during development

Question 11

How are dendritic spines involved in plasticity

Answer 11

Are found on dendrites, and increase stability/ new synapse formation
Turnover very quickly
- Spines labelled with PSD-95-eGFP, and move within minutes
- in vitro, 20% of PSDs turn over in 24h in hippocampal neurons, likely to be an underestimation of in vivo

Dendritic spines are stabilised by actin which decrease length and mobility, but increase width and stability

Question 12

How is actin polymerisation regulated?

Fischer 2000

Answer 12

AMPA and NMDA receptor activation reduces spine mobility and increases stability

Fischer 2000: selected dendritic spines with variation
Actin-eGFP imaged to investigate turnover

• NMDA receptor activation reduced mobility
• Inhibition with APV still reduced mobility (?role of AMPA)
• AMPA receptor activation reduced mobility
• Inhibition with CNQX did not affect motility

Question 13

Ionic basis of actin polymerisation by AMPA/NMDA

Answer 13

Due to AMPA-mediated Na+ influx and NMDA mediated Ca2+ influx
-Actin polymerisation correlates strongly with Sodium molarity
Blocking Na+ channels with TTX has no effect, it is mediated by AMPA receptors

Question 14

GluR2 subunit expression and spine stability

Passafaro 2003

Answer 14

GluR2 subunit expression associated with reduced Ca2+ permeability

• GluR2 Overexpression: Filopodia co-localised with PSD-95 were longer and more dense than controls
• N terminal domain mutation eliminated these effects

Question 15

PNNs in schizophrenia

Pantazopolous 2010

Answer 15

Massive increases in CSPG+ glial cells in amygdalic nuceli and entorhinal cortex assc. with reductions in PNNs
-ECM-glial interactions may contribute to disturbances in neuronal migration and neurotransmission

Question 16

PNNs in SCI

Answer 16

Lesion-dependent PNN changes may occur

• Decreases in Aggrecan, Neurocan, Phosphacan after traumatic brain injury
• Decreased Aggrecan after SC hemisection injury

Question 17

Features of SCI secondary injury

Answer 17

Glial infiltration (microglia, astrocytes)
Cytokine upregulation
CSPG secretion from astrocytes

Question 18

Factors that prevent regeneration after SCI

Answer 18

CSPGs
-Prevent regrowth into area, formation of dystrophic endbulbs

Semaphorin 3A (Sema3A): binds to PNNs and reduces neurite outgrowth in adult DRG cultures

Contact signalling between EphB-2 expressing fibroblasts and EphrinB2 expressing astrocytes
-Segregated following injury causing segregation

Question 19

Evidence that PNNs reduce lateral mobility of Receptors

Answer 19

Spines labelled for GlurR1, PNNs labelled for HABP-658
GluR1 labelled areas mostly devoid of HABP (‘holes’)

Removal of PNN increased lateral mobility of GluR1 receptors on Fluorescence Loss in Photobleaching (FLIP) imaging

Question 20

Chondroitinase ABC treatment in SCI

Answer 20

Improves neuron growth through scar, increases distal connections and fibre counts

• Improved beam walking, grid walking
• Functional recovery- pellet retrieval
• Increases neurite length following CSPG+Sema3A

Question 21

Issues with chABC treatments

Answer 21

Breaks down all types of CSPG, include types that may be beneficial
Destroys surrounding loose ECM
May increase sprouting of normal neurons- inducing new circuitry organisation
Would require multiple injections for human use and induce a significant immune response

Question 22

BDNF and Plasticity

Answer 22

BDNF is secreted in exercise
Has positive roles on differentiation, neurite extension, synaptic plasticity
mRNA upregulated following 7 days exercise training in the hippocampus

Question 23

BDNF and Actin Polymerisation

Answer 23

BDNF- binds TrkB receptors which have an actin-binding cytoplasmic domain, increasing spine stability
Ca2+ influx assc. with NMDA-mediated stability also causes NF-kB activation, transcribing more BDNF- positive feedback loop

Question 24

Link between exercise and Plasticity

Lungh and Sale 2015

Answer 24

Exercise vs. inactive control testing monocular deprivation

• Short term plasticity (measuring mean phase duration) robustly enhanced in exercise group
• May be mediated by a change in GABA inhibition, affecting excitation/inhibition balance in the visual cortex

Question 25

chABC treatment and Monocular Deprivation

Answer 25

chABC injection into cortex

• Causes shift in Contralateral bias index (shift towards ipsilateral, non-deprived eye)
• Similar performance to as seen in the critical period compared to adults

Question 26

Link protein targetting

Evidence- MD, digit innervation, NOR, memory+ age

Answer 26

Link protein expression controlled by Crtl1 gene
KO of Crtl1 reduces PNN formation in visual cortex on IHC
Dark rearing of animals also significantly reduced Crtl1; increased upon light

MD: contralateral bias reduced
Digit innervation: increased projection of Cholera toxin-labelled neurons from injured digit back to cuneate nucleus
NOR: Crtl1 KO mice performed better in NOR test at 24/48h
-Not improved further by chABC injection, suggesting link proteins are just as critical as CSPGs in PNNs

Age: Recovered age-associated memory deficits better in KOs

Question 27

chABC in tauopathies

Answer 27

P301S mutants/ AAV-injected mice used as tauopathy/AD model

• chABC improved object recognition after 1 week; but returned to normal after 5
• Restored synaptic transmission changes and improved LTD in perirhinal cortex
• Did not affect tauopathy progression in terms of Western blot mRNA

Question 28

Fear memory and chABC treatment

Answer 28

chABC in basolateral amygdala allowed erasure of fear extinction memory