Axonal Regeneration

Question 1

Describe the mechanisms of the the CNS injury pathway

Answer 1

CNS injury -> membrane depolarisation -> Ca2+ influx -(kinases + signalling}-> different genes on and off

Macrophages recruited and astrocytes + microglia proliferate-> release of chrondoitin sulfate and proteoglycans (CSPG)-> glial scar formation

Myelinated fibre disrup -> (MAG, OMgp, Nogo) -> RhoA -> growth cone collapse

Question 2

Describe the PNS injury pathway

Answer 2

NGF enters through the break in the membrane and is endocytosed -> retrogradely transported -> activates transcription factors for regenerative promoting genes

PNS signalling -> upregulates CREB and downregulates RhoA -> axonal regeneration

Question 3

How astrocytes cause growth cone collapse

Answer 3

Astrocytes release CSPG -> PTP-sigma receptor/Nogo receptor -> RhoA ->…-> growth cone collapse

Question 4

How oligodendrocytes cause growth cone collapse

Answer 4

Damaged oligodendrocytes -> myelin proteins exposed (Nogo, MAG, OMgp) -> RhoA ->…->growth cone collapse

Question 5

Main molecular pathways and mechanisms underpinning regenerative failure?

Answer 5

Extrinsic Signals

• astrocytes (CSPG)
• oligodendrocytes (MAG, OMgp, Nogo)
• ephrins/semaphorins

Intrinsic Signals

• neurotrophin
• integrin receptor
• retinoic acid
• mTOR

Question 6

How ephrin/semaphorin cause growth cone collapse

Answer 6

Ephrin/semaphorin -> RhoA -> … -> growth cone collapse

Question 7

How Retinoic acid contributes to axonal growth patterns

Answer 7

Retinoic acid (binds to intranuclear RA receptor) -> promotes intrinsic growth and inhibits inhibitory pathways ]- it represses gene activation of Lingo-1 (Nogo complex co-receptor), resulting in a downregulation of pathways that would normally cause growth cone collapse

Question 8

Intrinsic axonal growth signals for targeted therapies?

Answer 8

• neurotrophins
• integrin receptor
• retinoic acid
• mTOR

Question 9

Neurotrophin MoA?

Answer 9

Trk receptor -> cAMP -> PKA -CREBphosph-> pro-regen genes

NOTE: IL-6 works via a similar pathway
Injury -> inflamm -> IL-6 -> Gp130-Jak dimer -> STAT 3 phosph -> pro-regen genes

NB: could try and promote Trk signalling OR increase IL-6 expression

Question 10

Integrin receptor MoA?

Answer 10

Intracellular cascade (FAK) -> promotes axonal growth

NB: could try to overexpress integrin receptors

Question 11

Comparing CNS in different parts and PNS regarding regenerative ability

Answer 11

PNS > CNS (dorsal columns) > CNS (corticospinal tract)

Question 12

List some examples of how regenerative neuroscience is trying to promote axonal growth

Answer 12

Nogo antibodies, chrondoitinases, cAMP analogues/phosphdiesterase inhibitors, taxols, PTEN deletion, Sox11 overexpression, RhoA inhibitors, Nogo receptor knockout, epigenetic modification, histone deacetylase inhibitors (HDAC3)

Question 13

Chrondoitinases mechanism?

Answer 13

Chrondoitinases: dissolves CSPG, axons grow past lesion inhibits glial scar]- administered by lentivirus

Question 14

CAMP analogues/phosphodiesterase inhibitors mechanism?

Answer 14

cAMP analogues/phosphodiesterase inhibitors: stimulate growth cone remodelling (lack specificity)

Question 15

Taxols mechanism?

Answer 15

Taxols: stabilises microtubules, prevents growth cone collapse, reduces glial cell motility and reduces glial scarring,

Question 16

PTEN deletion mechanism?

Answer 16

PTEN deletion: PTEN inhibits mTOR, increased corticospinal tract (CST) axon regen

Question 17

Sox11 overexpression mechanism?

Answer 17

Sox11 overexpression: increased CST axon growth

Question 18

Epigenetic modification mechanism for regenerative neuroscience?

Answer 18

Epigenetic modification: relax histones to let transcription factors access DNA for axonal regeneration (in neurodevelopment histones are relaxed)

PCAF promotes acetylation in the PNS (not CNS)

After injury, retrograde transport of NGF/MEK/ERK activates PCAF

This relaxes histones and accesses pro-regenerative genes (in PCAF overexpression, axons grow past lesion site)

Question 19

Histone deacetylase inhibitors (HDAC3) mechanism?

Answer 19

Histone deacetylase inhibitors (HDAC3)

Dephosphorylation of HDAC3 reduces its activity

This allows transcription factors to access pro-regenerative genes]- promotes outgrowth of dorsal root ganglion neurons

Question 20

Goals of regenerative neuroscience?

Answer 20

Inhibit inhibitory signals of axonal growth

Promote pro-regenerative growth signals

Inhibit glial scar formation]- this is neuroprotective for isolating the lesion but is a molecular barrier for regeneration

Replace lost cells

Question 21

Examples of regenerative success?

Answer 21

• new axons grow around lesion/through lesion
• sprouting from transected axon and preserved axon (e.g. sprouting from lesioned to other supraspinal axons, sprouting to propriospinal relays, sprouting of unlesioned propriospinal axons, sprouting of remaining supraspinal axons -> regeneration)
• distal lesioned axons (Wallerian) degenerates: reconnectivity brings functional recovery
• different regen capacity: PNS> CNS (DCs)> CNS (CST)