Linking Molecular, Electrical And Anatomical Properties Of Human Epileptic Brain

Epilepsy is a common neurological disorder of recurrent unprovoked seizures. It affects almost 1% of the world population. Although there is a wide range of anti-epileptic drugs (AEDs) available, they only treat the seizure symptoms and do not cure the disease itself. The poor role of AEDs can be attributed to the lack of knowledge of exact mechanisms and networks that produce epileptic activities in the neocortex. At present, the best cure for epilepsy is surgical removal of electrically localized epileptic brain tissue. Surgically removed brain tissue presents an excellent opportunity to discover the molecular and cellular basis of human epilepsy. Patients with epilepsy have both seizures as well as another type of abnormal electrical wave form occurring in between seizures, and referred to as `interictal\u27 spikes. Interictal spikes occur far more frequently than seizures and in most epilepsy cases, cannot be associated with a clinical symptom. There is growing evidence supporting the role of interictal spikes in seizures and epilepsy. We have taken a high-throughput genomics approach and identified a consistent group of differentially expressed genes implicating MAPK/CREB signaling, immediate early genes (IEGs), and synaptic plasticity genes in regions of high spiking. Bioinformatic analyses showed a number of clusters within the MAPK/CREB genes that include both activators and inhibitors of MAPK signaling. DUSP4, a member of a family of dual specificity phosphatases function by inhibiting both isoforms of ERK1/2, is one of the most potent inhibitors of MAPK/CREB signaling. DUSP4 was significantly upregulated in high spiking areas, raising an important question as to why both activators and inhibitors are induced in high spiking areas. In situ hybridization of serial brain sections shows that DUSP4 is expressed in discrete micro-domains in the superficial neocortical layers and is inversely related to expression of di-phosphoERK, phosphoCREB, EGR1, and DUSP6, suggesting that DUSP4 activation in regions of high interictal activity directly inhibits the spread of electrical activity in these focal regions. These studies utilizing high throughput genomic studies from human neocortex provide great insight on the role and importance of interictal spiking and associated molecular changes and begin to define both mechanistic and spatial roles of MAPK signaling in neocortical epilepsy and have the potential to produce novel therapeutics. Furthermore, we have developed a software to quantify both the timing and spatial spread of intracranial spikes on long-term subdural electrographical recordings as a means to understand the relationship between spike spread and anatomical structure of the anatomical brain. We found that spikes occur in multiple channels of the subdural electrode grid within a hundred milliseconds and used these events to study the spatial and temporal patterns of interictal spike spread across cortical sites. Three different patterns of spike propagation were observed including 1) spread to electrodes on the same gyrus, 2) spread to neighboring electrodes but separated by a sulcus, and 3) Spread to distant electrodes (separated by multiple sulci). Interestingly, we found that the velocity of spread to distant electrodes was much higher compared to spikes that spread to adjacent electrodes, often on the same gyrus. This suggests at least two distinct conduction velocities through grey matter (slow) and white matter (fast). Using the datasets from five patients, we developed an equation to predict the pathway of spike propagation that incorporates both components of gray matter and white matter conduction for a given spike. These studies help our understanding of neocortical epilepsy and warrant further studies to determine whether patterns of interictal spike propagation could possibly play a role in surgical decision-making and improve epilepsy surgery outcomes