- Top of page
- Mechanisms of Epileptogenesis
- Biomarkers of Epileptogenesis
- Targeting Epileptogenesis
- A Neurometabolic Synthesis
For several decades, both in vitro and in vivo models of seizures and epilepsy have been employed to unravel the molecular and cellular mechanisms underlying the occurrence of spontaneous recurrent seizures (SRS)—the defining hallmark of the epileptic brain. However, despite great advances in our understanding of seizure genesis, investigators have yet to develop reliable biomarkers and surrogate markers of the epileptogenic process. Sadly, the pathogenic mechanisms that produce the epileptic condition, especially after precipitating events such as head trauma, inflammation, or prolonged febrile convulsions, are poorly understood. A major challenge has been the inherent complexity and heterogeneity of known epileptic syndromes and the differential genetic susceptibilities exhibited by patients at risk. Therefore, it is unlikely that there is only one fundamental pathophysiologic mechanism shared by all the epilepsies. Identification of antiepileptogenesis targets has been an overarching goal over the last decade, as current anticonvulsant medications appear to influence only the acute process of ictogenesis. Clearly, there is an urgent need to develop novel therapeutic interventions that are disease modifying—therapies that either completely or partially prevent the emergence of SRS. An important secondary goal is to develop new treatments that can also lessen the burden of epilepsy comorbidities (e.g., cognitive impairment, mood disorders) by preventing or reducing the deleterious changes during the epileptogenic process. This review summarizes novel antiepileptogenesis targets that were critically discussed at the XIth Workshop on the Neurobiology of Epilepsy (WONOEP XI) meeting in Grottaferrata, Italy. Further, emerging neurometabolic links among several target mechanisms and highlights of the panel discussion are presented.
To date, pharmacologic strategies to mitigate or prevent epileptogenesis in humans—most notably after head injury—have proven to be ineffective (Holtkamp & Meierkord, 2007; Löscher & Brandt, 2010), despite vast laboratory evidence that many anticonvulsant medications possess neuroprotective properties (Pitkänen, 2002; White, 2002). This failure is often believed to be a consequence of several factors, including species differences, inappropriate timing of intervention, focus on nonessential molecular targets, differential genetic susceptibility, and perhaps long-term neurotoxicity of drugs (Pitkänen & Lukasiuk, 2011). The same issues apply to the stroke field but perhaps even more so, as dozens of prospective randomized clinical trials—rationally based on specific molecular targets validated in animal models—have failed to show significant effects in humans (Engel, 2001; Löscher & Brandt, 2010). Another important consideration is that the molecular targets that have traditionally been studied may be more relevant to ictogenesis (i.e., induction of an acute seizure) than to epileptogenesis, the latter which reflects a variable process resulting in an enduring state of spontaneous recurrent seizures. Such limitations highlight the need to identify novel molecular targets for antiepileptogenesis.
The unpredictability of seizure activity, and indeed an increasingly recognized progressive disease state that defines many forms of epilepsy, poses numerous challenges toward our understanding of pathophysiology, efforts in designing and implementing laboratory investigations, and conducting controlled clinical studies. It is important to recognize that there are highly variable outcomes after epileptogenic brain insults, and little is known about the factors that govern such differential pathways. Moreover, the modifying influences of environment, attempts at treatment, and comorbid medical problems (intrinsic or acquired) serve to complicate an already confounding set of processes. Against this frustrating backdrop, there are growing international efforts to develop reliable biomarkers and surrogate disease markers of epileptogenesis, which may help clinicians identify patients at high risk following a brain insult (whether they be febrile seizures, status epilepticus [SE], head trauma, or brain inflammation). Such markers will also undoubtedly prove immensely useful in assessing patients with underlying genetic etiologies and in those individuals where the underlying cause(s) of spontaneous unprovoked seizures remain a mystery. However, the overall utility of biomarkers and surrogate markers rests in large measure on how these reflect the true pathophysiology of the epileptogenic process itself. And it should be duly noted that like the heterogeneous nature of the epilepsies, a singular mechanism or molecular pathway may not be applicable to epileptogenesis in general. Investigators should bear in mind this critical notion when attempting to advance experimental therapeutics designed to arrest or modify the epileptogenic disease process.
At present, there is a wide range of animal models of epileptogenesis, but many of them are likely not clinically meaningful to a large extent when viewed from the standpoint of precipitating causes. Other than traumatic brain injury, insults such as electrical brain stimulation (e.g., kindling) or exposure to chemoconvulsants (e.g., pilocarpine) or excitotoxins (e.g., kainic acid) generally do not occur in humans who develop epilepsy. As such, the question can and should be raised as to the relevance of molecular and cellular changes observed in such models. Specifically, the spectrum of alterations reported during the presumed epileptogenesis process in animal models include neuronal injury and cell death, axonal and dendritic plasticity, presynaptic and postsynaptic modifications, neurogenesis, neuroinflammation, glial cell activation, vascular damage and angiogenesis, disruption of extracellular matrix integrity, as well as structural (i.e., subunit) and functional changes in ion channels properties (Pitkänen et al., 2007).
Summarized below is a composite review of presentations made at the 11th Workshop on the Neurobiology of Epilepsy (WONOEP XI) on the topic of antiepileptogenesis targets, which was organized by the Neurobiology Commission of the International League Against Epilepsy (ILAE) in Grottaferrata, Italy (August 23–27, 2011). Both older and newer molecular targets and approaches were discussed, and all were framed with novel therapeutic perspectives. These include α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors, thrombospondin receptors, epigenetic regulation, granular “hub” cells, neurotrophic factors, erythropoietin (EPO)–derived peptide mimetics, ketogenic diet, and bumetanide derivatives as potential treatment approaches as well as diffusion tensor (magnetic resonance) imaging (DTI) to monitor treatment effects.
We propose that true antiepileptogenesis targets should be validated by their ability to either completely prevent the emergence of spontaneous recurrent seizures, delay their onset (i.e., partial prevention), or by modifying seizure frequency, duration, and/or severity. Broadly speaking, such changes would fall under the rubric of disease or syndrome modification, implying that one or more therapeutic interventions based on such targets have some critical effects on the epileptogenic process(es). Following brief overviews of the diverse array of potential targets, an attempt at weaving the seemingly unrelated topics into a common “neurometabolic” fabric is made.
A Neurometabolic Synthesis
- Top of page
- Mechanisms of Epileptogenesis
- Biomarkers of Epileptogenesis
- Targeting Epileptogenesis
- A Neurometabolic Synthesis
Although the mechanisms and molecular targets described above are seemingly disparate, many if not all (including those not covered in this review) may have in part an underlying basis in neurometabolism—specifically, in that the diverse array of metabolic substrates, enzymes, and biochemical pathways inherent in virtually every cell undoubtedly impacts cellular processes in a manner that can influence the actions of ion channels, neurotrophic factors, and even epigenetic regulation.
The prototypic metabolic treatment is the KD, which is clinically effective in reducing seizure activity in patients who fail to respond to conventional and even newer anticonvulsant medications. Despite the lack of a detailed understanding of how the KD works to control seizures (Masino & Rho, 2012), there is mounting evidence that the KD, ketones, and PUFAs are broadly neuroprotective (Gasior et al., 2006; Maalouf et al., 2009) through a multiplicity of effects, including, but not limited to a reduction in oxidative stress (Sullivan et al., 2004; Milder & Patel, 2011), enhancement in purinergic signaling (Masino et al., 2011), and increases in bioenergetic substrates such as ATP (Bough et al., 2006; Kim Do et al., 2010). These actions suggest that certain substrates that define such metabolism-based treatments might ameliorate a number of neurologic disease states, and indeed there is growing support for this concept (Stafstrom & Rho, 2012). With respect to epileptogenesis, the early evidence for a disease-modifying effect of the KD and its related treatments appears suggestive but not conclusive (Muller-Schwarze et al., 1999; Todorova et al., 2000; Bough et al., 2003; Stafstrom et al., 2009; Linard et al., 2010; Schwartzkroin et al., 2010; Jiang et al., 2012).
Pertinent to this review, although it remains uncertain whether PUFAs exert direct anticonvulsant effects (given the lack of concordance between animal and human studies), it is possible that omega-3 fatty acid derivatives may be antiepileptogenic. Indeed, it was recently reported that neuroprotectin D1 (NPD1), a derivative of docosahexaenoic acid (DHA) significantly attenuated kindling progression and hippocampal hyperexcitability (Musto et al., 2011). Along a similar vein, in an attempt to mirror the glucose restriction that is a hallmark feature of the KD, Garriga-Canut et al. (2006) demonstrated that 2-deoxyglucose, an inhibitor of phosphoglucose isomerase (a key enzyme in the glycolytic pathway, which prevents the conversion of glucose-6-phosphate to fructose-6-phosphate) prevented kindling epileptogenesis in rats. Specifically, 2-DG suppressed seizure-induced increases in BDNF and TrkB, which are mediated by the transcriptional repressor neuron restrictive silencing factor (NRSF) and its nicotinamide adenine dinucleotide (NADH)–sensitive corepressor carboxy-terminal binding protein (CtBP) (Garriga-Canut et al., 2006).
At this juncture, the effects of epigenetic DNA modification on neuronal excitability and on epileptogenesis are unknown. However, preliminary investigations in Wistar rats subjected to pilocarpine-induced SE have shown that a non–calorie-restricted KD reduced seizure frequency and antagonized aberrant DNA methylation (K. Kobow, unpublished data). A distinct correlation between seizure frequency and DNA promoter methylation pattern was seen in these studies.
It has been stated above that epileptogenesis may involve aberrant recruitment of mechanisms underlying activity-dependent synaptic potentiation, in part through effects involving AMPA-receptor modulation. Of interest, there are conflicting data regarding whether a KD impacts hippocampal LTP. Koranda et al. (2011) assessed the effects of theta-burst stimulation of dentate gyrus using in vivo recordings in freely behaving normal rats, and found that KD treatment significantly diminished LTP responses. This is in contrast to the study by Thio et al. (2010) who examined paired-pulse modulation (PPM) and LTP in the medial perforant path of rats in vivo and found no significant effects with KD treatment. Moreover, these investigators found that locomotor activity and behavior in a conditioned fear test were similarly unaffected. Although the reasons for this discrepancy remain unclear, it is clear that further studies examining the metabolic effects on synaptic plasticity and integrity are warranted.
Because the cation chloride cotransporters NKCC1 and KCC2 are prominently involved in the GABA-depolarizing effect in immature neurons (see above), and given that pharmacologic antagonism of NKCC1 has been shown to render anticonvulsant effects (Dzhala et al., 2005, 2008), a logical question to ask is whether the anticonvulsant KD might alter NKCC1 or KCC2 expression. An initial study examining expression levels of both cation-chloride cotransporters using immunocytochemical methods failed to reveal a difference in the number or intensity of labeled cells in hippocampus of adolescent rats fed a KD (Gomez-Lira et al., 2011). Therefore, it appears that one could rule out such cotransporters as mechanistic targets of KD action, but it should be noted that for many such studies, these negative findings were made in normal (and not epileptic) brain.
If seizures increase neurogenesis, and preventing this might yield antiepileptogenic and anticonvulsant effects, then another straightforward question would be whether the KD alters neurogenesis. Again, there are opposing data. In normal adult rats, KD treatment for 4 weeks did not change the number of bromodeoxyuridine (BrdU) immunoreactive cells in dentate gyrus (Strandberg et al., 2008), whereas the KD was found to significantly increase the number of BrdU-positive cells (and thus enhance neurogenesis) following kainate-induced seizures in mice (Kwon et al., 2008).
In summary, although there are at present only scant data regarding the effects of metabolism-based treatments such as the KD on the panoply of mechanisms and targets that have up to now been implicated in both ictogenesis and epileptogenesis (Masino & Rho, 2012), it would not be altogether surprising to witness a steadily increasing number of publications that link specific antiepileptogenesis targets to one or more metabolic substrates, enzymes or pathways—not just whether the KD affects any of these targets. The fundamental role of neurometabolism in cellular homeostasis and disease states is becoming increasingly appreciated, but there is much more that needs to be investigated.