Inflammation in Epilepsy Flashcards
Define ‘seizure’
A sudden, transitory and uncontrolled disruption of brain activity
What is epilepsy?
• Condition in which recurrent seizures occur → seizures are a symptom of epilepsy
• A seizure is defined as a sudden, transitory and uncontrolled disruption of brain activity
• Around 40 different types of seizure
• Affects people of all ages:
− ~0,5 million in the UK
− 1 in 20 will have a seizure in their lifetime
• Epilepsy is the most common chronic neurological condition
• The condition is chronic, but the symptoms are acute
• Often has a childhood onset – although febrile seizures are common in childhood and these are not epilepsy
Describe the 5 types of primary seizure.
- Generalised seizures are produced by electrical impulses throughout the entire brain
- Absence → short loss of consciousness (few seconds) with new or no symptoms. The patient typically interrupts an activity and stares blankly. They begin and end abruptly and may occur several times a day. Patients usually not aware they are having a seizure, but may be aware of “losing time”
- Myoclonic → short, jerking movings of parts of the body. Sometimes described to feel like brief electrical shocks.
- Tonic → going stiff and falling, but without convulsions
- Tonic clonic → stiff and falling, with convulsions
- Atonic → falling limply to the ground
Describe the 3 types of partial seizure
- Produced by electrical impulses in a small part of the brain
- Simple → activity while the person is aware and conscious. Jerking, muscle rigidy, spasms, head-turning, unsual sensations. Memory or emotional disturbances
- Complex → Activity that results in impairment of awareness. Automatisms such as lip smacking, chewing, fidgeting, walking and other repetitive involuntary movements
- Partial with secondary generalization → Symtoms initiall associated with a preservation of consciousness that then evolves into loss of consciousness and convulsions.
Describe the aetiology of epilepsy
• Epilepsy is the result of complex genetic/environmental factors: − Abnormal brain wiring − Chemical (neurotransmitter inbalances) − Ion channel dysfunctions − Abnormal connections made when attempting to repair an injury • Idiopathic – no apparent cause • Crytogenic – a likely cause but not able to be identified • Symptomaic – cause identified − Vascular – 10% − Congenital – 8% − Trauma – 6% − Tumour – 4% − Degenerative – 4% − Infection – 3% • Post-stroke seizures are common
Describe the pathogenesis of epilepsy
- Dominating theory for decades is that there is an imbalance between excitatory vs inhibitory transmission → causes hypersensitive neurons
- They fire more intensely, more often, and with greater amplitude than normal neurons
- Normally, GABA keeps things in check – but a reduction in GABA leads to increased neuronal activity
- These neurons are therefore easily activated by hyperthermia, hypoxia, hypoglycaemia, hyponatremia, sensory stimulation (eg, bright lights) and certain sleep phases → these are the classical warning triggers of epilepsy – they are epileptogenic
How is epilepsy currently treated?
Aims:
• Block repetitive neuronal firing
• Block synchronization of neuronal discharges
• Block propagation of seizure
Strategies:
• Modification of ion conductancies
• Increase inhibitor (GABAergic) transmission
• Decrease excitatory (glutaminergic) activity
- For some, treatments are very effective and they wont have another seizure
- Most anti-epileptics are old, no breakthroughs in 25 yeasr
- Many need to be taken throughout life – including during pregnancy! All she can do is maybe lower the dose
• Anti-epileptic drugs only aim to treat the symptoms – there is no cure
• NICE recommendation is for monotherapy:
− Carbamazepine (Increased Na+ channel inactivation, increased GABA)
− Ethosuximide (decrease T-type Ca2+ channels)
− Levetiracetam (blocks synaptic vesicle release)
− Sodium valproate (Increases GABA)
• 70% of patients could have seizure control with these, however currently only ~50% do
• 30% are drug refractory
• Surgery can be an option
− Henry Molaison had intractable epilepsy that was found to be localized to his left and right medial temporal lobes
− A surgeon performed the lobectomy, but also removed the hipoocampus and adjacent structures – the hippocampus appeared non-functional because it looked atrophic
− The surgery was successful in treating the epilepsy. but he developed severe anterograde amnesia – couldn’t commit new events to his explicit memory
− This showed us the hippocampus is important in memory
• Existing AEDs have side effects → memory problems, fatigue, depression, nausea, visual problems
What are some animal models of epilepsy?
The most common way is to cause seizure in animals:
• Administratino of convulsant compounds
− Kainic acid (glutamate agonist – increases excitation)
− Bicuculline (GABA antaonist – removes inhibition)
− Pilocarpine (muscarinic agonist – increases excitation)
• Electric stimulation
− Kindling → commonly used model in which the duration of the induced seizures increases after seizures are induced repeatedly. Use to study the effects of repeated seizures. Repeated stimulation lowers the threshold for more seizures to occur
However, these are acute, and we know epilepsy is chronic. Allow study of seizure, not epilepsy
• Animals with ‘epilepsy’
− Genetic absence epilepsy rat strain (GAERS)
− DBA audogenic mouse – has a seizure in response to an audio stimulus
How do we assess seizures in the experimental models?
• Induce the seizure, then measure it via:
• EEG → measure the brain waves to measure the seizure diretly
• Behavioural assessments → can categorise according to the Racine classification:
− Stage 1 – mouth and facial movements
− Stage 2 – head nodding
− Stage 3 – forelimb clonus
− Stage 4 – rearing
− Stage 5 – loss of postural control
− Stage 6 – death
• These are instantly transplatable to the clinic
For evidence for a role of inflammation in epilepsy. what would we want to see?
- Changes in expression of inflammatory mediators → ELISA or IHC
- Microglia/astrocyte activation
- Changes in adhesion molecules/leukocyte infiltration
- Adminstration of an inflammatory mediators → proconvulsant
- Inhibition of an inflammatory mediator → anti-convulsant
- Genetic association of inflammatory mediators with increased risk of epilepsy
Describe the changes in expression of inflammatory mediators seen in epilepsy
Cytokines:
Mouse model:
• Vezzani is a key researcher in inflammation in epilepsy
• Following injection of kainic acid, you get a rapid increase in IL-1B 3 hours later – this increase is much faster than seen in stroke
• Also rapid increase in IL-6, TNFa by 6 hours
• IL-1RA also increases a little later
Clinical data:
• Cytokines measured in the CSF 24hrs post tonic-clonic seizure
• Changes noted in IL-6, bit not IL-1, TNFa or NGF
• When you look in the plasma, the change in IL-6 only seems to be found in temporal lobe epilepsy
− Data from a biopsy from someone who has undergone surgery as treatment shows that in TLE, there is clear neuronal damage (hippocampal sclerosis)
− There isn’t this clear damage in other forms of epilepsy
− There was only IL-1 expression if the patient had sclerosis
− Both the complement pathway and the plasminogen system are activated in a sclerotic hippocampus in TLE patients
− Since complement, IL-1B and plasminogen activators can affect the permability of the BBB, we can speculate a re-inforcing feedback loop between these pathways. which may contribute to BBB breakdown
miRNA:
• miR-146 can be induced by different pro-inflammatory stimuli such as IL-1B and TNFa
• It can modulate innate immunity through regulation of TLR signaling and cytokine responses
• This miRNA is upregulated in TLE as well as in experimental models of epilepsy
TLR4:
• Overexpression in neurons and astrocytes found in TLE and focal cortical dysplasia
• Its ligand HMGB1 is a pro-inflammatory danger signal, relased from activated or damaged neurons
• Lowers threshold to seizures
Describe seizure-induced microglial and astrocyte activation.
These experiments involved pilocarpine-induced status epilepticus
• Using OX-42 as a label for microglia, and GFAP as a label for astrocytes
• In the acute phase, you get the onset of acute seizures. If you drive the pathway hard enough – believed there is a period of change in the brain known as epileptogenesis – the result of this is chronic epilepsy, where the animals will get spontaneous seizures
• In the control, everything looks normal
• In the acute phase, you get amoeboid microglia
• There is then a change back to the ramified state – but they are not the same as the control, they have been changed
• The activation of both microglia and astrocytes is associated with the induction of pro-inflammatory pathways in TLE.
• Activation of these cells and concomitant induction of inflammatory pathways observed in patients with tuberous sclerosis complex – a major cause of pediatric epilepsy
→ We don’t know what this means, as the paper didn’t look at it functionally. But there are changes
Describe changes in adhesion molecules and leukocyte infiltraiton in epilepsy
Changes in adhesion molecules:
• 6hrs after pilocarpine admin in mouse, ICAMs, VCAMs, Selectins increase
• This goes away following anti-convulsants (diazepam)
Changes in leukocytes:
• Following pilocarpine treatment, 100% of leukocytes are rolling or arrested
• 5-20 days after systemic pilocarpine,blocking leukocyte adhesion with anti-adhesion molecule antibodies reduces the number of convulsions
• So, stopping leukocyte interactions with the epithelium has anti-epileptic properties
Model:
• Proposed that in the pilocarpine model of epilepsy, you have cerebrovascular inflammation, whereby you have extravasation of leukocytes into the brain, which has an effect on the BBB, allowing leakage of proteins from the blood which will lead to seizure
• So the suggestion in this paper (Keen & Holmes) is that the cerebrovascular inflammation was triggering the epilepsy.
Describe what happens with administration of inflammatory mediators in epilepsy.
• Vezzani et al, showed that with IL-1 injectino along with Bicuculine induced epilepsy:
− Onset to both clonus and tonus is faster
− Time spent in clonus and tonus is longer
Describe what happens with inhibition of inflammatory mediators in epilepsy.
- Vezzani et al, showed adminstration of IL-1RA increased the time taken for onset of clonus and donus, and decreased the time spent in clonus and tonus
- Mice overexpressing the soluble form of human IL-1RA in astrocytes were intrinsically resistant to seizures
If IL-1B can cause epilepsy, one would argue that if you inhibited the processin gof IL-1B it would be anti-convulsant:
• Rezazni et al showed that in caspase 1 KO mice, the time taken for onset of a seizure was increased, and the duration and number of seizure episodes was decreased
• It is argued that it is better to inhibit caspase 1 rather than knocking it out, as if it was KO since birth, may be some compensatory mechanisms in place
• Effects of caspase 1 inhibition is the same
Does this have clinical relevance?
Clinical trial of caspase 1 inhibitor:
• 6 week randomized, double blind, multicenter, placebo-controlled study, phase IIa
• Subjects 18-64 years old with treatment-resistant partial epilepsy
• VX-765 or placebo given
• Initial data analysis suggested some positive results
• None of the placebo group were seizure-free, but 20% treated group were seizure free
• This was in a treatment-resistant group – patients who have not responded to anything previously – notoriously difficult to see a response in these people.
• These results led to a phase IIb trial → but was terminated because the drug wouldn’t have made money – this is because it was only tested in treatment resistant patients, so would have only been able to be used in these patients
• But could have provided important proof of concept if they carried on
TLR4:
• Seen how overexpression of TLR4 and its ligand HMGB1 in microglia and astrocytes is found
• Mice with defective TLR4 signalling show significant delay in onset of seizures and were intrinsically resistant to seizure activity
• Notably – both the IL-1 and TLR signaling pathways are similar
