Role of endosomal toll-like receptors in epilepsy
Epilepsy is a common disorder affecting about 60 million people worldwide. The population of epilepsy patients who cannot achieve seizure freedom has remained stubbornly fixed at around 30% despite the introduction of new therapies in recent years. The only way to stop the development of epilepsy is...
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|Summary:||Epilepsy is a common disorder affecting about 60 million people worldwide. The population of epilepsy patients who cannot achieve seizure freedom has remained stubbornly fixed at around 30% despite the introduction of new therapies in recent years. The only way to stop the development of epilepsy is to prevent an injury. Epilepsy is caused by myriad factors and is characterized by recurrent and spontaneous seizures, increased mortality rate, and decreased social interaction and quality of life (Henshall et al. 2016). The harmful effects include disruption of the developmental process and neuronal degeneration (Yehezkel Ben-Ari and Holmes 2006). The most affected region due to epilepsy is the hippocampus, a part of the limbic system. There are no treatments that can prevent epilepsy; hence, there is a clear need for better anti-epileptic remedies.
The Innate immune system acts as the first line of defense against foreign intruders (Akira 2003). Toll-like receptors (TLRs) are a part of the immune system and were first discovered in Drosophila melanogaster. TLRs are involved in early host defense against pathogens, and they recognize a pathogen- or damage-associated molecular pattern (PAMPs/DAMPs). TLRs can also identify phagocytes such as neutrophils, macrophages, and dendritic cells (Akira 2003). They play a role in innate immunity, and TLR signaling leads to inflammatory gene expression changes. The first report of TLRs in epilepsy was by Turrin and Rivest (Turrin and Rivest 2004). All studies related to TLRs in epilepsy have been confined to the cell surface TLRs, e.g., TLRs 2 and 4 (Maroso et al. 2010).
TLRs 3, 7, and 9 are expressed intracellularly, whereas TLRs 1, 2, 4 are expressed on the cell surface. TLR3 recognizes double-stranded RNA (dsRNA) and is associated with viral infection. TLR7 recognizes single-stranded RNA virus. TLR9 recognizes unmethylated CpG DNA motifs, characteristics of DNA viruses, and prokaryotic genomes. TLR4 is most well-known for recognizing lipopolysaccharide (LPS), a component present in many bacteria. Only TLRs 2 and 4 have been implicated in both experimental and human epilepsy, and the endosomal TLRs (eTLRs) are yet to be studied.
Our research group recently discovered, serendipitously, that mice lacking certain TLRs have spontaneous seizures. This information led us to hypothesize that TLR deficiency causes epilepsy. This hypothesis was tested by determining: 1) which of these TLRs is/are responsible for epilepsy, and 2) whether TLR activation can prevent epilepsy. In the thesis, I used two different animal models of epilepsy: a) perforant path stimulation (PPS), and b) systemic injection of kainate and lorazepam (KaL).
I found that TLRs are upregulated in the hippocampus during epileptogenesis and chronic epilepsy phases, as validated in both animal models using qPCR. I found upregulation of mRNA in associated cytokines and chemokines. I also showed that the TLR proteins are upregulated during chronic epilepsy. Lastly, I knocked down the expression of TLRs 3 and 7, and found that TLR3/7 knockdown did not have any effect on seizure reduction.
To summarize, this project revealed that the TLR mRNA and protein expression are upregulated during epileptogenesis and chronic epilepsy. Knocking down the TLRs using siRNA did not have any effect on the development of epilepsy or inactivation of spontaneous seizures. The originality of the work lies in the fact that we are, to the best of our knowledge, the first to use a phenotype-driven approach to elucidate the role of (as yet unexplored) TLRs in epilepsy.|
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