Breaking Balance: Synaptic Interneuron Properties Shift the E-I Balance in FCD I Epilepsy

Abstract

Net Synaptic Drive of Fast-Spiking Interneurons is Inverted Towards Inhibition in Human FCD I Epilepsy

Cho E, Kwon J, Lee G, Shin J, Lee H, Lee SH, Chung CK, Yoon J, Ho WK. Nat Commun. 2024;15(1):6683. doi: 10.1038/s41467-024-51065-7. PMID: 39107293; PMCID: PMC11303528.

Focal cortical dysplasia type I (FCD I) is the most common cause of pharmacoresistant epilepsy with the poorest prognosis. To understand the epileptogenic mechanisms of FCD I, we obtained tissue resected from patients with FCD I epilepsy, and from tumor patients as control. Using whole-cell patch clamp in acute human brain slices, we investigated the cellular properties of fast-spiking interneurons (FSINs) and pyramidal neurons within the ictal onset zone. In FCD I epilepsy, FSINs exhibited lower firing rates from slower repolarization and action potential broadening, while PNs had increased firing. Importantly, excitatory synaptic drive of FSINs increased progressively with the scale of cortical activation as a general property across species, but this relationship was inverted toward net inhibition in FCD I epilepsy. Further comparison with intracranial electroencephalography from the same patients revealed that the spatial extent of pathological high-frequency oscillations was associated with synaptic events at FSINs.

Commentary

For too long epilepsy was believed to be a single disorder, defined by the presence of seizures. However, we now understand that while seizures are the observable event that allow one to diagnose epilepsy, in reality, epilepsy is a degenerate disorder, with many different mechanisms that can give rise to the seizures that characterize it.^1,2 We are therefore now tasked with identifying the many subtypes of epilepsy and exploring their differences and commonalities in order to understand the multitude of ways that epilepsy manifests itself. In their recent article, Cho et al ³ advance this goal by examining the mechanisms that underly a common, but not well understood, subtype of epilepsy: focal cortical dysplasia (FCD) type I.

FCD represents a group of pathologies associated with abnormal cortical development and is often associated with drug-resistant epilepsy. ⁴ Importantly, FCD itself is heterogeneous and has been further dived into 3 subtypes: FCD I, II, and III. ⁵ FCD I and II both are defined by isolated lesions with cortical dyslamination, but in FCD II, the neurons in the lesioned area also dysmorphic. FCD is classified as type III if other lesions are also present. While the classification of FCD into type I or II is defined by the presence (or lack thereof) of dysmorphic neurons, it is important that the 2 subtypes of FCD also manifest differently in their clinical presentation. Individuals with FCD II tend to present with seizures earlier in life but have better surgical outcomes than those with FCD I. ⁶ Recent work has linked genetic mutations in the mammalian target of rapamycin (mTOR) pathway to the development of FCD II. ⁴ Other work has begun to examine the genetic underpinnings of FCD I, finding that mTOR does not appear to be involved in this subtype and FCD I likely involves more genetic heterogeneity. ⁷ It has thus been surmized that the mechanisms of epileptogenesis are distinct for FCD I.

Cho et al, therefore decided to investigate potential mechanisms that might upset the excitation-inhibition (E-I) balance in FCD I by examining the electrophysiological properties of fast-spiking interneurons (FSIN) and pyramidal neurons (PN) in human tissue from the neocortical layer 2/3 of the ictal onset zone of epileptic patients who had undergone resective surgery. They also obtained healthy control tissue from patients without a history of epilepsy who had undergone tumor resection for comparison. Using a whole-patch clamp set up, they found that in FCD I, FSINs had lower firing rates and longer duration of action potentials (APs) than in control, despite having similar AP thresholds. The FSIN firing rate correlated with the half-width of the AP which in turn correlated with the maximum rate of repolarization, and these relationships were consistent in both FCD I and control.

When examining PNs, the authors found that in both FCD I and control tissue, PNs had the same resting membrane potential, but FCD I PNs had a higher input resistance and, for small current injections, responded with a higher firing frequency. While they found no relationship between AP properties and PN firing rate, the PN firing rate correlated with the input resistance, causing the authors to conclude that FSINs and PNs are differentially affected in FCD I.

The authors then investigated the role of synaptic properties and how synaptic conductances might differ between FCD I and control tissue in FSINs and PNs. Using spontaneous activity to measure synaptic conductance, they found that for both cell types and in both FCD I and control, the inhibitory conductance G_I was higher than the excitatory conductance G_E, and the ratio of G_E/G_I was not different between the 2 conditions. However, when using min and max stimulation protocols to calculate synaptic conductances, the situation changed. In FSINs from control tissue, as the stimulation increased, G_E increased, becoming larger than G_I with maximum stimulation. However, in FSINs from FCD I tissue, the increase in G_E was accompanied by a similar increase in G_I, such that the G_E/G_I ratio remained in favor of net inhibition. The authors thus concluded that in control settings, FSINs experience increased excitation in response to increased cortical activity, but in FCD I, these cells remain inhibited. Interestingly, when examining PNs, the authors found that in both control and FCD I tissue, the G_E/G_I ratio remained in favor of net inhibition even as stimulation increased.

The deep dive into how specific synaptic properties are changed in this subtype of epilepsy is incredibly important for the development of precision medicine. As the authors discuss, voltage-gated potassium channels are an active area of interest to target in developing treatments for focal epilepsy, with some clinical trials already underway. ⁸ Given that these channels are responsible for repolarization, and that repolarization was affected in FCD I FSINs, it is important to investigate what specific subunits might be altered in FCD I such that these specific subunits could be targeted for therapeutic intervention. Also, it will be important as future clinical trials are developed that outcomes are not averaged over multiple subtypes of epilepsy. For example, in this case of voltage-gated potassium channels, it could be that different subtypes of focal epilepsy are mechanistically linked to different subunits of these channels and while the intervention might work very well in FCD II, it could completely fail in FCD I. If the results are averaged, it is possible to miss an incredibly effective intervention for a single subtype.

This gives rise to another important consideration: What mechanisms are common across multiple types of epilepsy? What mechanisms are specific to only certain subtypes? The seizure isn’t the only point of degeneracy in epilepsy. Changes in the E-I balance can give rise to seizures, but there are multiple ways that the E-I balance can be broken. However, there are also multiple ways in which the E-I balance can be restored. In some cases, we might be currently unable to alter a biological mechanism. However, if 2 different biological mechanisms lead to the same imbalance of E-I, perhaps a similar treatment might work in both cases if we target fixing the E-I imbalance as opposed to the biological mechanism. Regardless, it is work like that of Cho et al that is essential in furthering our quest for these answers as we strive to understand the mechanisms that give rise to specific subtypes of epilepsy.