Identifying Blood-Brain Barrier Pathologies and Novel Therapeutic Targets in Epilepsy and Neuropsychiatric Disorders

Abstract

The blood-brain barrier (BBB) is vital for maintaining homeostasis of the neural microenvironment. Tight junctions composed of the transmembrane proteins claudin-5 and occludin contribute to the function of this barrier by limiting paracellular diffusion through endothelial cells. Claudin-5 is the most enriched protein at the tight junction and its expression is required for size-selective regulation of BBB integrity. Altered expression of claudin-5 and dysfunction of the BBB has been implicated in many neurological conditions such as Alzheimer¿s disease, multiple sclerosis, and traumatic brain injury. This thesis expands the current knowledge by reporting BBB disruption in both psychiatric disorders and epilepsy and propose this disruption of the BBB is responsible for the close association between these and other neurological conditions as co-morbidities.

Dysregulation of tight junction proteins is identified in the neuropsychiatric disorders, schizophrenia, bipolar disorder and major depression, with downregulation of claudin-5 in post-mortem hippocampal brain tissue of schizophrenia and major depression patients. A bidirectional relationship is also reported between BBB integrity and epilepsy. BBB dysfunction is a hallmark feature of human temporal lobe epilepsy, with increased permeability of the BBB as observed by dynamic contrast-enhanced MRI and reduced expression of claudin-5 in surgically resected brain tissue. Diminished expression of claudin-5 is also demonstrated in the acute kainic acid mouse model of seizures. It is also demonstrated here that directly regulating expression of claudin-5 alters seizure susceptibility. Prolonged suppression of claudin-5 in mice leads to the development of spontaneous seizures while claudin-5 heterozygosity exacerbates the severity of seizures induced by kainic acid.

BBB stabilising drugs and specifically, modulating claudin-5 expression presents a new therapeutic target for neurological conditions. This thesis provides a proof of concept for the viability of restoring claudin-5 expression as a therapy in epilepsy as reversal of claudin-5 suppression in mice rescues seizure phenotypes. Further exploring therapeutic avenues in epilepsy, preliminary data here provides the groundwork for targeting neuronal cell death as a therapy in epilepsy. Neuronal cell death is a critical pathology in the epileptic hippocampus and this death is mediated by the mitogen activated protein kinase, dual-leucine zipper kinase (DLK). Inhibiting DLK activation offers a viable therapeutic target to protect against loss of neurons. In vitro experiments demonstrate the activity of a kinase dead DLK isoform. This dominant-negative construct binds to DLK, inhibiting its activation and enhancing its degradation.