Download Mitochondria and Epilepsy

Mitochondria and Epilepsy
Major Research interests:
Dysfunction of the mitochondrial respiratory chain associated with epilepsy
Mitochondrial oxidative phosphorylation provides the major source of ATP in neurons. It
consists of five multienzyme complexes located in the mitochondrial inner membrane
(Figure). The complexes I to IV are oxidoreductases which participate in the transfer of
reducing equivalents from NADH and FADH2 to oxygen and create an electrochemical
proton gradient across the mitochondrial inner membrane. Complex V – the F0F1-ATPase uses this proton gradient for the synthesis of ATP. Defects of oxidative phosphorylation in the
CNS are the characteristic sign of mitochondrial encephalopathies. In a broad variety of these
diseases epileptic seizures have been observed. An overview of the most common
mitochondrial disorders presenting with an epileptic phenotype is given in the Table. Most of
them are associated with mutations in the autonomous mitochondrial DNA, consisting of 13
polypeptide genes, a 16S and a 12S rRNA gene and 22 tRNA genes. A well known
mitochondrial disorder with an epileptic phenotype is the MERRF (myoclonus epilepsy with
’ragged red fibers’) syndrome which has been associated with mutations in the mitochondrial
tRNALys gene. However, as shown in the Table, other mitochondrial DNA mutations
predominantly localised in the mitochondrial tRNA genes for lysin, serin, leucin, isoleucin or
cystein have been also associated with epileptic phenotypes. These mutations affect the
protein biosynthesis of all mitochondrial-encoded subunits of the following complexes of the
mitochondrial oxidative phosphorylation pathway: complex I (NADH:CoQ oxidoreductase,
containing 7 mitochondrial-encoded subunits), complex III (CoQH2:cytochrome c
oxidoreductase, containing 1 mitochondrial-encoded subunit), complex IV (cytochrome c
oxidase, containing 3 mitochondrial-encoded subunits) and complex V (F0F1-ATPase,
containing 2 mitochondrial-encoded subunits). Quite rarely, also mutations in polypeptidecoding mitochondrial genes have been reported in patients with epilepsy – in the ATPase 6
gene, in the CO III gene and in the ND 1 gene. We have recently identified a novel mutation
in the CO I gene associated with Epilepsia patialis continua.
The large variation in the clinical phenotype, even for a given mutation, is a well
known feature of mitochondrial diseases. It is therefore very likely that the distribution of the
mitochondrial defect within the CNS is the responsible factor which determines the
association of a certain mutation with epilepsy. Thus KSS is almost a white matter disorder
affecting preferentially brainstem tegmentum, white matter of cerebellum and cerebrum,
cervical spinal cord, basal ganglia, and diencephalon. On the other hand, MELAS is
characterised by foci of necrosis, which are predominantly localised in the cerebral cortex and
also in the hippocampus – a highly epileptogenic area, whereas MERRF involves
preferentially the inferior olivary nucleus, the cerebellar dentate nucleus, the red necleus and
the pontine tegmentum – structures being implicated in the genesis of myoclonus.
In contrast to the relatively rare mitochondrial encephalopathies being associated with
mtDNA mutations epilepsy is a frequent neurological disorder usually well controlled by
presently available drugs. However, 20 to 30% of patients do not experience seizure control
with available medication. The majority of these patients suffer from focal epilepsies which
frequently develop subsequently to brain trauma, complicated febrile convulsions, status
epilepticus, ischemic lesions and brain tumours. The areas of epileptogenesis in these cases
are usually characterised by cell loss. It is well documented that during seizures both nerve
cells and glia undergo necrotic and apoptotic cell death. Neuropathological investigations
have repeatedly pointed to a similarity between ischemic and seizure related alterations of
neurons characterised by swollen and often disrupted mitochondria. In patients with
Ammon’s horn sclerosis mitochondrial ultrastructural pathology was described as
characteristic feature of hilar neurons. In this context is noteworthy to mention, that in
addition to the pathological abnormalities also functional defects of mitochondria have been
reported in the areas of epileptogenesis. Thus, we observed a severe impairment of respiratory
chain complex I activity for CA3 neurons in the hippocampus from patients with Ammon’s
horn sclerosis and in the parahippocampal gyrus of patients with parahippocampal lesions. In
these reports the mitochondrial abnormalities have been observed only close to or directly in
the epileptic focus, while the investigated surrounding brain tissue (e.g. the parahippocampal
gyrus of patients with a clearly pronounced hippocampal pathology and a hippocampal
seizure focus) showed now mitochondrial pathology.
Mitochondria as potential trigger for neuronal cell death observed in epilepsy
Prolonged seizures (status epilepticus) induced in experimental models by kainic acid or
pilocarpine are known to activate programmed cell death mechanisms. This cell death
observed also in human epilepsy is one of the most important aspects of epileptogenesis. In
the hippocampus the loss of CA1, CA3 and CA4 pyramidal neurons, with relative sparing of
the granular neurons of the dentate gyrus and some types of interneurones, is the
histopathological hallmark of Ammon’s horn sclerosis. The mechanism that underlies this
regional selectivity remains to be elucidated, some data point to differential expression of
proaportotic and antiapoptotic genes. The probably most important factor preceding neuronal
cell death after status epilepticus is the increased level of reactive oxygen species observed in
various models of experimental epilepsy – kainate-induced hippocampal damage, pilocarpine
treatment and low Mg2+-induced epileptiform activity in brain slices and slice cultures.
Mitochondria are known to be the most important source of production of reactive oxygen
species. Moreover, increased production of reactive oxygen species is a feature of partially
respiratory chain-inhibited mitochondria and it is noteworthy to mention in this context that a
severe impairment of respiratory chain complex I activity is present in the areas of
epileptogenesis - in CA3 neurons of the hippocampus from patients with Ammon’s horn
sclerosis and in the parahippocampal gyrus of patients with parahippocampal lesions. Similar
observations we made recently in the vulnerable CA1 and CA3 hippocampal subfields of
pilocarpine-treated chronic epileptic rats. As potential cause of the detected respiratory chain
impairment we could delineate a decrease of the mtDNA copy number. This finding indicates
a possible substantial role of oxygen radicals in causing neuronal mtDNA damage occurring
selectively in the areas of epileptogenesis.
How mitochondrial dysfunction can alter neuronal excitability ?
Beside alterations of mitochondrial substrate oxidation and ATP synthesis due to diseaseassociated mutations discussed in detail before, also the direct partial inhibition of enzymes of
mitochondrial respiratory chain – of cytochrome c oxidase by cyanide, and of succinate
dehydrogenase by 3-nitropropionic acid – evoke seizures. Potential direct links between the
observed impairment of mitochondrial function and the increased neuronal excitability
causing epileptiform activity are (i) decreased intracellular ATP levels and (ii) alterations of
neuronal calcium homeostasis. A relatively high impact of neuronal ATP levels can be
postulated since epileptic seizures are observed in Leigh syndrome patients harbouring the
mutations T8993G and T8993C in the ATPase 6 gene. Under these conditions mitochondria
still have a high membrane potential enabling normal ion transport. Therefore, for cybrids
with the T8993G NARP mutation normal mitochondrial calcium handling properties at
decreased cellular ATP levels were observed. It has to be mentioned that mitochondrial
oxidative phosphorylation provides the major source of ATP in neurons and adequate ATP
levels are essential to maintain the neuronal plasma membrane potential via the sodiumpotassium ATPase which consumes about 40% of the energy. Therefore, the decreased
neuronal plasma membrane potential is most likely responsible for epileptic seizures observed
in Leigh syndrome patients harbouring ATPase 6 gene mutations.
On the other hand it is well established that mitochondria are an important intracellular
Ca2+ sequestration system. Especially due to this feature, mitochondria are believed to
modulate neuronal excitability and synaptic transmission which is altered in epilepsy. In
agreement with this concept in kainate-treated chronic epileptic rats impaired oxidative
phosphorylation due to Ca2+ cycling at the inner membrane of hippocampal mitochondria has
been demonstrated by us. Similarly, impaired cellular Ca2+ homeostasis due to substantial
alterations of mitochondrial Ca2+ handling was the predominant feature of cybrid cells
harbouring the mitochondrial T8356C mutation being associated with MERRF.
Brain energy metabolism as potential target of neuroprotective strategies
For a certain number of neurodegenerative diseases with established mitochondrial pathology,
like amyotrophic lateral sclerosis and Chorea Huntington neuroprotective strategies have been
suggested. The proposed treatment is buffering of neuronal energy levels by systemic creatine
administration. The supplemented creatine passes the blood-brain barrier and increases the
total pool of phosphocreatine / creatine available for buffering of the neuronal ATP levels by
creatine kinase. Creatine supplementation has been shown to protect motor neurons in a
transgenic animal model of amyotrophic lateral sclerosis and striatal neurons in an animal
model of Huntingston’s disease. Our NMR spectroscopic data on patients with amyotrophic
lateral sclerosis showed upon creatine supplementation increased N-acetyl aspartate levels in
the motor cortex of these patients, suggesting at a potential improvement of neuronal energy
levels. Interestingly, the buffering of brain energy levels with creatine seems to be effective
not only in the mentioned neurodegenerative disorders but also in hypoxia-induced or
traumatic brain injury. Moreover, 3 g/kg creatine were observed to reduce hypoxia-induced
seizures in rat and rabbit pups. The neuroprotective effect of the creatine treatment in human
pathology remains, however, still to be shown.
Selected recent publications:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
Kunz, W.S., Kuznetsov, A.V., Clark, J.F., Tracey I. & Elger, C.E. (1999) Metabolic
consequences of the cytochrome c oxidase deficiency in brain of copper-deficient Movbr
mice. J. Neurochem. 72, 1580-1585
Kunz, W.S., Goussakov, I.V., Beck, H. & Elger, C.E. (1999) Altered mitochondrial
oxidative phosphorylation in hippokampal slices of kainate-treated rats. Brain Res. 826,
236-242
Kudin, A., Vielhaber, S., Beck, H., Elger, C.E. & Kunz, W.S. (1999) Quantitative
investigation of mitochondrial function in single hippocampal slices: A novel
application of high-resolution respirometry and laser-excited fluorescence spectroscopy.
Brain Res. Prot. 4, 93-98
Vielhaber, S., Kunz, D., Winkler, K., Wiedemann, F.R., Kirches, E., Feistner, H.,
Heinze, H.-J., Elger, C.E., Schubert, W. & Kunz, W.S. (2000) Mitochondrial DNA
abnormalities in skeletal muscle of patients with sporadic amyotrophic lateral sclerosis.
Brain 123, 1339-1348
Schröder, R., Vielhaber, S., Wiedemann, F.R., Kornblum, C., Papassotiropoulos, A.,
Broich, P., Zierz, S., Elger, C.E., Reichmann, H., Seibel, P., Klockgether, T. & Kunz,
W.S. (2000) New insights in the metabolic consequences of large scale mtDNA
deletions: A quantitative analysis of biochemical, morphological and genetic findings in
human skeletal muscle. J. Neuropath. Exp. Neurol. 59, 353-360
Kunz, W.S., Kudin, A.P., Vielhaber, S., Blümcke, I., Zuschratter, W.,Schramm, J.,
Beck, H. & Elger, C.E. (2000) Mitochondrial complex I deficiency in the epileptic
focus of patients with temporal lobe epilepsy. Ann. Neurol. 48, 766-773
Kunz, W.S., Kudin, A., Vielhaber, S., Elger, C.E., Attardi, G. & Villani, G. (2000) Flux
control of cytochrome c oxidase in human skeletal muscle. J. Biol. Chem. 275, 2774127745
Kunz, W.S. (2001) Control of oxidative phosphorylation in skeletal muscle. Biochim.
Biophys. Acta (Bioenergetics) 1504, 12-20
Debska,G., May, R., Kicinska, A., Szewczyk, A., Elger, C.E. & Kunz, W.S. (2001)
Potassium channel openers depolarize hippocampal mitochondria. Brain Res. 892, 4250
Vielhaber, S., Schröder, R., Winkler, K., Weis, S., Sailer, M., Feistner, H., Heinze, H.J., Schröder, J.M.& Kunz, W.S. (2001) Defective mitochondrial oxidative
phosphorylation in myopathies with tubular aggregates originating from sarcoplasmic
reticulum. J. Neuropath. Exp. Neurol. 60, 1032-1040
Vielhaber, S., Kaufmann, J., Kanowski, M., Sailer, M., Feistner, H., Tempelmann, C.,
Elger, C.E., Heinze, H.-J. & Kunz, W.S. (2001) Effect of creatine supplementation on
metabolite levels in ALS motor cortices. Exp. Neurol. 172, 377-382
12.
13.
14.
15.
16.
17.
Kunz, W.S. (2002) The role of mitochondria in epileptogenesis. Curr. Opin. Neurol.
15, 179-184
Kunz , D., Winkler, K.,Elger, C.E. & Kunz, W.S. (2002) Functional imaging of
mitochondrial redox state. Method. Enzymol. 352, 135-150
Kudin, A.P., Kudina, T.A.,Seyfried, J., Vielhaber, S., Beck, H., Elger, C.E. & Kunz,
W.S. (2002) Seizure-dependent modulation of mitochondrial oxidative phosphorylation
in rat hippocampus. Eur. J. Neurosci. 15, 1105-1114
Varlamov, D.A., Kudin, A.P., Vielhaber, S., Schröder, R., Sassen, R., Becker, A., Kunz,
D., Haug, K., Rebstock, J., Heils, A., Elger, C.E. & Kunz, W.S. (2002) Metabolic
consequences of a novel missense mutation of the mtDNA COI gene. Hum. Mol.
Genet. 11, 1797-1805
Kudin, A.P., Vielhaber, S., Elger, C.E. & Kunz, W.S. (2002) Differences in flux
control and reserve capacity of cytochrome c oxidase (COX) in human skeletal muscle
and brain suggest different metabolic effects of mild COX deficiencies. Biocomplexity
1, in press
Vielhaber, S., Varlamov, D.A., Kudina, T.A., Schröder, R., Kappes-Horn, K., Elger,
C.E., Seibel, M., Seibel, P. & Kunz, W.S. (2002) Expression pattern of mitochondrial
respiratory chain enzymes in skeletal muscle of patients harbouring the A3243G point
mutation or large scale deletions of mitochondrial DNA. J. Neuropath. Exp. Neurol., in
press