Identifying targets for preventing epilepsy using systems biology

Abstract

While there are a plethora of medications that block seizures, these same drugs have little effect on preventing or curing epilepsy. This suggests that the molecular pathways for epileptogenesis are distinct from those that produce acute seizures and therefore will require the identification of novel truly 'antiepileptic' therapeutics. Identification and testing of potential antiepileptic drug targets first in animal models and then in humans is thus becoming an important next step in the battle against epilepsy. In focal forms of human epilepsy the battle, however, is complicated by the large and varied types of brain abnormalities capable of producing a state of chronic, recurrent seizures. Unfortunately, once the epileptic state develops, it often persists to produce a life-long seizure disorder that can only be suppressed by anticonvulsant medications, and cured only in some through surgical resection of the seizure focus. While deductive approaches to drug target identification use our current state of knowledge, based mostly on animal models of epileptogenesis, a growing reductionist approach often referred to as systems biology takes advantage of newer high-throughput technologies to profile large numbers and types of molecules simultaneously. Some of these approaches, such as functional genomics, proteomics, and metabolomics have been undertaken in both human and animal epileptic brain tissues and are beginning to hone in on new therapeutic targets. While these methods are highly sensitive, this same sensitivity also produces a high rate of false positives due to variables other than those of interest. The experimental design, therefore, needs to be tightly controlled to reduce these unintended results that can be misleading. Most importantly, epileptogenic targets need to be validated in animal models of epileptogenesis, so that, if successful, these new methods have the potential to identify unbiased, important new therapeutics.

Figure 1. A model for targeted epileptogenesis drug development

Following a wide array of brain insults, epilepsy develops over a prolonged latent period. During this latent period both cellular, networking, and underlying molecular events occur that create an often permanent state of recurrent seizures. Current medications work mostly as anticonvulsants preventing seizures, but have little proven effects on epileptogenesis. Newer, truly anti-epileptogenic drugs that target either acute or more chronic molecular events during the latent period are needed. One hypothesis, is that feedback between cellular, electrical, and molecular events grows the epileptic focus over time until a threshold is reached that is capable of producing clinical seizures.

Figure 2. A systems biology model for high-throughput studies of epilepsy

Within a cell, such as the neuron shown here, genetic differences as well as differences in gene expression (mRNA), protein expression, and small molecule expression (such as neurotransmitters) characterize the epileptic focus. These individual molecular systems must also be placed within the contexts of larger systems including organ, tissue, and cell type. The complexity of this system creates one of the greatest challenges for the study and interpretation of high-throughput molecular profiling methods in epilepsy.

Figure 3. Computational workflow linking human epilepsy surgery to systems biology

In order to keep track of hundreds of variables that will affect the outcome of human epileptic tissue studies, we have developed a database the links all aspects of the pre-surgical, surgical, and tissue work. These include neuropsychology, electrophysiology, imaging, tissue location, and a number of high-throughput molecular studies derived from these tissues.

Figure 4. Model of epileptogenesis from genes to pathways identified from functional genomics studies

Based on the pattern of gene activations in human neocortical regions of seizure onset, this model describes how ongoing epileptic activities could create a state of heightened signaling through a number of pathways including the CREB transcription factor [59]. Some of these pathways that are known to activate CREB signaling such as protein kinase A (PKA), calcium calmodulin kinase II (CAMKII), or mitogen activated protein kinase (MAPK) could be targeted for anti-epileptic drug development.