The role of astrocytes in SCN8A epileptic encephalopathy and the role of SCN8A in lymphocyte development and function

SCN8A epileptic encephalopathy is a rare, devastating early onset form of epilepsy caused by
mutations in the voltage-gated sodium channel (VGSC) Nav1.6 encoded by the gene SCN8A. SCN8A
epileptic encephalopathy is characterized by an early onset between birth and 18 months of age followed by developmental regression with an increased risk of sudden unexpected death in epilepsy (SUDEP). Most research focus on SCN8A encephalopathy has centered on the role of Nav1.6 in neurons as well as neuronal mechanisms leading to seizure manifestation in SCN8A model mice and human patients. However, there is a growing appreciation for the contribution of non-neuronal cells to the development and the manifestation and maintenance of the seizure phenotype. In particular, astrocytes are now recognized as key players in seizures and epilepsy and deciphering astrocyte-mediated mechanisms contributing to seizures has been outlined as a research goal by the NINDS in their 2020 Epilepsy Benchmarks research focus. In addition to the involvement of astrocytes, it has been reported that infections and seizures co-occur in SCN8A epileptic patients. Currently there are no studies addressing an immune component to SCN8A epileptic encephalopathy. Evidence from other types of epilepsy indicate that pro-inflammatory molecules, such as IL-1β and TNFα are seizure promoting and dampening inflammation and resolving infections ameliorate seizure severity and reduce seizure occurrence.
In this thesis I investigated the role of glial cells in SCN8A epileptic encephalopathy as well as
characterized alterations in the immune system in mice carrying a human derived point mutation in the
SCN8A gene. I provide evidence that astrocytes in spontaneously seizing SCN8A mutant mice become
reactive as indicated by a dramatic increase in the expression of the cytoskeletal protein glial fibrillary
acidic protein (GFAP). Surprisingly, despite widespread astrocyte reactivity in the cortex and
hippocampus, I detected no microglial activation based on microscopy and Sholl analysis of microglia. Due to the presence of astrocyte reactivity, I investigated whether several key functions of astrocytes might be impaired. Astrocyte potassium channels are necessary to maintain the proper extracellular potassium concentration necessary for normal neuronal function. Using electrophysiology, I show that astrocytes in spontaneously seizing mice have reduced potassium channel function. Astrocytes are also tasked with the removal of glutamate and its enzymatic degradation to prevent excess excitation. I assessed astrocytic expression of the glutamate degrading enzyme, glutamine synthetase, and found that in spontaneously seizing mice astrocytes exhibited reduced expression of this enzyme. Together these data provide evidence for astrocyte dysfunction in spontaneously seizing SCN8A mutant mice and are suggestive that impaired astrocyte function may contribute to the pathology of SCN8A epileptic encephalopathy.
While exploring the role of microglia in SCN8A mutant mice I serendipitously found alterations in
the immune system. SCN8A mutant mice exhibit reduced size of immune organs, such as the spleen and thymus. Immune organ size was dependent on the copy number of mutant SCN8A with size from largest to smallest as follows: wildtype, heterozygous then homozygous SCN8A mutant mice. Mice homozygous for the mutation in SCN8A exhibited an altered B cell compartment and bone marrow chimera experiments demonstrate impaired T cell activation following immunization with the model antigen NPOVA in alum. Together these data demonstrate that the immune system in SCN8A mutant mice is altered. It is important to stress that these are preliminary findings, but it is tempting to speculate that the deficit in T cell activation in response to model antigen could explain the prevalence of infections in SCN8A epileptic encephalopathy patients and why some of these patients succumb to lower respiratory tract infections (Atanasoska et al., 2018; Gardella et al., 2018; Johannesen et al., 2018). Altogether this thesis investigates several neglected, but still very important non-neuronal aspects of SCN8A epileptic encephalopathy that may help to reveal novel treatments for this devastating disease beyond the current ion channel and neuron focused anti-epileptic drugs.