Understanding physiologic changes in ataxia

Vikram G. Shakkottai

Cells of the cerebellum

Physiologic features of cerebellar neurons

• Pacemaker neurons: Purkinje and DCN neurons

• Neurotransmitter systems: GABA projection neurons (Purkinje neurons)

Glutamatergic projection neurons (DCN neurons)

• Interneurons: GABA: Golgi, Basket, Stellate, Lugaro. Glutamate: UBN

Cerebellar circuitry

• Cerebellar Input:

Climbing fibers: Inferior olive

Mossy fibers: All other input

• Cerebellar output:

DCN projections

Vestibulocerebellum- direct projections from PC to vestibular nuclei

How does the cerebellum work?

Why bother understanding Physiologic changes in Ataxia?

• Mutations in ion channels. Neuronal dysfunction from perturbed channel activity

• Perturbations in ion channel physiology in the absence of a primary channel defect

• Novel therapeutic targets for symptomatic treatment of ataxia.

Why study cerebellar physiology?Ataxic channelopathies

Disorder

SCA6SCA13SCA15Episodic ataxia 1Episodic ataxia 2Episodic ataxia 5SMEI

Ion channel mutation (gene/protein)

CACNA1A/Cav2.1KCNC3/Kv3.3ITPR1/IP3 receptorKCNA1/Kv1.1CACNA1A/Cav2.1CACNB4/Cav2.1 auxiliary subunitSCN1A/Nav1.1

Ataxias associated with channel dysfunction without channel mutations

Disorder

SCA27SCA5DRPLA Episodic ataxia 6Paraneoplastic cerebellar ataxia

Ion channel involved

Nav1.6Ionotropic glutamate receptorsIonotropic glutamate receptors, GABAA receptors

Ionotropic glutamate receptorsCav2.1

Ataxia from ion-channel mutations

Ataxic channelopathies

Disorder Ion channel mutation (gene/protein)

Type of mutation

SCA6 CACNA1A/Cav2.1 ?Haploinsufficiency/?Gain of function

SCA13 KCNC3/Kv3.3 Dominant-negative

SCA15 ITPR1/IP3 receptor Dominant negative

Episodic ataxia 1 KCNA1/Kv1.1 Dominant-negative

Episodic ataxia 2 CACNA1A/Cav2.1 Haploinsuffiency

Episodic ataxia 5 CACNB4/Cav2.1 Auxiliary subunit

?Haploinsufficiency/?Dominant negative

SMEI SCN1A/Nav1.1 Haploinsufficiency

No channel mutation but channels involved

Ataxias associated with channel dysfunction without channel mutations

Disorder Ion channel involved

SCA27 Nav1.6

SCA5 Ionotropic glutamate receptors

DRPLA Ionotropic glutamate receptors, GABAA receptors

Episodic ataxia 6 Ionotropic glutamate receptors

ParaneoplasticCerebellar ataxia

Cav2.1

SCA6

• CAG repeat/polyglutamine disorder

• Expansion of a polyglutamine tract in the carboxyl terminus (C-terminus) of the P/Q type voltage-gated calcium channel alpha subunit, Cav2.1

• A relatively “pure” cerebellar syndrome, SCA6 is notable for selective degeneration of cerebellar Purkinje neurons

• Precise mechanism by which the altered Cav2.1 protein causes ataxia remains uncertain

• Altered channel function (haploinsufficiency)?

• Production of an aberrant C-terminal fragment (gain of function)?

SCA6

SCA13

• Mutations in the KCNC3 gene encoding the voltage-gated potassium channel protein, Kv3.3

• Purkinje neurons in null mice display broader action potentials, fail to sustain high frequency firing in response to injected current, and have reduced burst frequency in response to climbing fiber stimulation

• The cerebellar atrophy seen in patients with SCA13 has been hypothesized to be due to increased calcium entry into Purkinje neurons secondary to the spike broadening seen in the null mice

Episodic ataxia 1 (EA1)

• Mutations in the voltage-gated potassium channel, KCNA1 (Kv1.1)

• Purkinje neurons show increased GABAergic inhibitory postsynaptic currents (IPSCs)

• Kv1.1 normally hyperpolarizes axon branch points of Basket neurons, preventing some action potentials from reaching the presynaptic terminal

• More action potentials to invade presynaptic terminal?

EA1wikipedia

How does acetazolamide work?

Basket cell showing axon branch points

EA2 and EA5

• Mutations in CACNA1A • EA2 is allelic with SCA6 and familial hemiplegic migraine• More than 20 CACNA1A mutations have been identified

which result in a truncated, nonfunctional protein• Attacks of ataxia and migraine-like symptoms that may

be precipitated by physical and emotional stress, coffee or alcohol

• Signs of cerebellar dysfunction can be present between the paroxysmal episodes

• Another form of episodic ataxia, EA5, is caused by mutations in CACNB4 which encodes an auxiliary beta subunit of Cav2.1

EA2

• Mutations in the P/Q-type voltage-gated calcium channel, Cav2.1, leads to reduced calcium current density

• Impaired synaptic transmission at the parallel fiber-Purkinje neuron synapse

• Reduced precision of intrinsic pacemaker firing by Purkinje neurons

EA2

• A calcium-activated potassium channel opener improves motor behavior in mice

• Acetazolamide prevents or attenuates attacks in 50% to 75% of patients.

• Acetazolamide-induced changes in intracellular pH and the resulting change in potassium channel conductance may explain the therapeutic effect of the drug

SCA27

• SCA27 was first described in a Dutch pedigree manifesting childhood-onset postural tremor and slowly progressive ataxia beginning in young adulthood

• Orofacial dyskinesias, postural limb tremor, disorders of executive function and non-verbal memory often occur

• Mutations in the fibroblast growth factor 14 (FGF14) gene cause SCA27

SCA27

• Although SCA27 is not caused by an ion channel mutation, the ataxic phenotype in this disorder results from perturbed expression of voltage-gated sodium channels in cerebellar neurons

• Most Purkinje neurons in a mouse model of SCA27 do not fire spontaneously

• There is reduced expression of Nav1.6 in Purkinje neurons

• Mutant FGF14 protein fails to interact with sodium channel subunits, and interferes with the interaction between wild-type FGF14 and voltage-gated sodium channel subunits in a dominant manner

Paraneoplastic cerebellar ataxia

• Cerebellar ataxia sometimes occurs in association with Lambert–Eaton myasthenic syndrome (LEMS)

• LEMS: immune-mediated disorder of the neuromuscular junction associated with antibodies directed against the voltage gated P/Q type calcium channel, Cav2.1

• Cerebellar symptoms were in a recent study present in 9% of 97 patients with LEMS, in nearly all cases associated with small cell lung cancer

• When applied to cerebellar slices, an antibody directed againt Cav2.1 reduced synaptic transmission at the parallel fiber–Purkinje cell synapse

• An anti-Cav2.1 antibody conferred an ataxic phenotype by passive transfer in mice

Paraneoplastic cerebellar ataxia

How about the degenerative ataxias?

• Neuronal loss does not explain all the phenotypic changes in the degenerative ataxias

• For example mouse models of SCA1 show a neurological phenotype prior to significant neuronal cell loss

• Expression of expanded (pathogenic) ataxin-3 in differentiated neural cells caused electrophysiologic dysfunction preceding the onset of nuclear inclusions and ultrastructural morphological changes

Postulated mechanism for altered physiology and ataxia