Part 1: Introduction
Summary of Cellular and Circuitry Mechanisms (Meera et al., 2016)
Pacemaking and IP3-mediated Ca2+ signalling are defining features of PN physiology
Mechanism involving slowly rising calcium levels reinforced by positive feedback elements provides
potential explanation for slowly progressing nature of the disease
Ca2+-activated SK potassium channels provide critical brake on PN excitability
o Chronically elevated basal Ca2+ could lead to slowed pacemaking and dysregulates the
expression and or post-translational modulation of ion channels required for pacemaking
o Results have implicated changes in BK-type Ca2+-active and A-type voltage-gated K+
channels
Slowed PN firing and elevated Ca2+ leads to saturated forms of circuit plasticity which would impair
motor learning and lead to imbalances in output of cerebellar circuitry
Argument: spinocerebellar ataxias are associated with neurodegenerative loss of Purkinje cells within the
cerebellum, leading to cerebellar atrophy
Role of Cerebellum: the cerebellum is essential for coordinating movement functions as a rapid,
corrective feedback loop that smooths and coordinates movements
Receives sensory information
Damage of the cerebellum (as seen in Ataxias) will cause aberrant eye movements,
dydiadochokinesia, dysmetria, intention tremor, and motor learning deficits
Purkinje cells are GABAergic inhibitory cells that are located within the cerebellum
When these cells slow or stop firing, their targets are excited
Highly intrinsically active – sustained activity does not require excitatory synaptic input but arises
because of concerted activity of ions channels expressed by PNs
Channels prompt PNs to generate their own regulate skiing activity – fire at approx. 40 Hz with
metronomic regularity (Raman and Bean, 1999)
Under these conditions, coefficiency of variation in inter-spike instantaneous frequencies are
typically less than 10% over long periods of time
PNs appear to have characteristic baseline firing rate
Optogenetic control of Purkinje cell excitability
o Zhang et al., 2010
o Lee and Matthews et al., 2015
Rapid, short latency arm movements triggered by brief PC inhibition
Conclusion: all ataxias results from damage to the cerebellum and interconnected regions of the nervous
system. Therefore, ataxia is common to all SCA. However, the exact neuropathological findings of each
SCA and their cause varies genetic heterogeneity
Argument: Alterations in spontaneous AP firing in Purkinje Neurons underlies basic features of
SCA
Sustained, regular PN firing is degraded in at least 6 mouse models of SCA and in other non-SCA-
related transgenic mouse models that exhibit behavioural ataxia
In PN-specific, human transgene model of SCA2 (pcp2-Atxn2127Q), progressive dysregulation of
transcriptional expression patterns + severity of behavioural ataxia track reduction of mean PN firing
rates over 8 month time course of disease progression (Hansen et al., 2013)
o Captures basic features of human disease (intact motor behaviour + normal cerebellar
morphology at birth followed by progression of motor dysfunction and PN loss in
later life)
PN-specific SCA2 model with smaller CAG repeat length (pcp2-ATXN2Q58) shows impaired firing
rates and less regular firing (Kasumu et al., 2012)
Progressive reduction in pacemaking seen in PN-specific models of SCA1 (Hourez et al., 2011), in
global YAC transgenic for SCA3 (Shakkottai et al., 2011) and in a B-III spectrin knockout mouse
model of SCA5 (Perkins et al., 2010)
o SCA5 model showed reduction in resurgent component in VG-Na+ current which plays a key
role in pacemaking
PN-specific SCA6 transgenic mouse line expressing C-terminal fragment, corresponding to exon 47,
of the P-type Ca2+ channel containing 27 polyQ repeats has been characterised (Mark et al., 2015)
, o PN firing rates are reduced
o Firing is irregular
Shown in a myriad of mouse lines utilising different transgenes, promoters, and SCA subtypes
pacemaking ability tightly correlates with behavioural ataxia
Degradation of physiological output preceded overt loss of PNs
o Suggest that reduced PN spiking output is common pathophysiological feature of SCA and
contributes to ataxic symptoms characteristic of disease
Argument: Reduced PN Pacemaking Contributes to Ataxia
1. Transgenic mouse lines where cerebellar genes are deleted show slowed PN firing rates and ataxic
phenotype
2. Moonwalker mouse line (point mutation in TRPC3 ion channel causing constitutive activation )
shows reduction in PN firing frequencies (Sekerkova et al., 2013)
3. PN-specific TSC1 KO mouse shows significant reductions with loss of both copies of TSC1 gene
(Tsai et al., 2012)
4. Both global and PN-specific lines where copies of RNA plsicing genes Rbfox1 and -2 have been
deleted show ataxic behaviour and reduced PN firing (Gehman et al., 2012)
5. These mouse models do not recapitulate the genetic forms of SCAs but the correspondence
between reduced PN intrinsic excitability and behavioural ataxia is supportive fo key role of this
electrophysiological feature plays in normal motor behaviour
Argument: What are the Circuit Consequence of Reduced PN Output
PN output is directed exclusively to deep cerebellar nucleus (DCN) and vestibular nucleus neurons
– they are GABAergic so tonic activity provides baseline inhibition of downstream neurons (some
are prmemotor neurons in the descending rubrospinal motor pathway)
If there is no compensation in downstream pathways, the loss of tonix inhibition from PNs would
cause increased cerebellar nuclear neuron excitability and increased motor drive
Optogenetic experiment where PN output is transiently silented show that cerebellar nucleus
neurons burst, diriving rapid movements (Heiney et al., 2014)
Optogenetic stimuli affecting PN firing drive associative learning at a behavioural level (Lee et al.,
2015)
o The stimuli drive activity-dependent synaptic plasticity resulting in parallel fibre-PN LTD
reduced excitatory drive from PFs (red burst)
o In DCN, it causes mossy fibre-DCN LTP increased excitatory drive onto DCN (blue burst)
o This would promote ectopic movement seen in optogenetic training (Lee et al., 2015)
o
o Meera et al., 2016
BUT, why are ectopic movements, dystonia and chorea not observed in SCA?
o Neurons downstream of PNs (cerebellar nucleus neurons, red nucleus neurons, thalamic
neurons) compensate for the increased excitability that results from reduction in PN
inhibition
