Authors :
Presenting Author: Victoria Ho, MD, PhD – UCLA
Ruth Tjondropurnomo, BS – UCLA
Jennifer Nguyen, BS – UCLA
Samantha Depew, BS – UCLA
Balko Eszter, PhD – University of Veterinary Medicine Budapest
Bence Racz, PhD – University of Veterinary Medicine Budapest
Peyman Golshani, MD, PhD – UCLA
Rationale:
Temporal lobe epilepsy (TLE) is the most common form of focal epilepsy and is often refractory to medical management. Epileptogenesis in TLE is believed to be triggered by a precipitating injury followed by maladaptive neurocircuit rewiring that results in a network predisposed to seizures. However, the mechanisms underlying these changes are not understood and there are currently no therapies to prevent epileptogenesis.
Methods:
We have taken a transcriptomic approach to elucidate the mechanisms underlying epileptogenesis in TLE. Epilepsy was induced using intraperitoneal injections of pilocarpine in wildtype mice. At 3-6 weeks post-induction, mice were monitored with EEG to ensure a 12-hour period of seizure-freedom prior to tissue collection in order to minimize effects of acute seizures on the transcriptome. Dorsal hippocampal tissue was extracted from epileptic and control mice and profiled using single nucleus sequencing. Follow-up studies were performed with immunohistochemistry (IHC), fluorescence in situ hybridization (FISH), and electron microscopy (EM).
Results:
A total of 134,929 nuclei passed quality control and were used for further analyses. All major cell types were represented and many were found to have genes differentially expressed between epilepsy and control samples. Notably, we identified a microglia subcluster that is abundant in epilepsy but nearly absent from control samples. These epilepsy-associated microglia (EAM) were enriched for genes including insulin-like growth factor 1 (Igf1) and myosin 1e (Myo1e), which showed the highest fold enrichment and were minimally expressed in other microglia clusters. Using dual IHC-FISH with these two genes as markers, we found that EAM are more numerous in epileptic hippocampi, especially around the CA1-3 pyramidal layers. We examined additional time points and found that EAM appear within a day of pilocarpine treatment, increase in density at 3-6 weeks, and approach control levels at 32-weeks post-induction. Additionally, EAM have less ramified morphologies and were often found in clusters. Using EM, microglia from epilepsy samples were found to have larger mitochondria and more abundant vesicles.
Conclusions:
We have identified a population of microglia that are present in epileptic hippocampi and absent from control. These microglia are less ramified, have larger mitochondria and more abundant vesicles, suggesting they are more actively engaged in phagocytosis. They are most abundant in the subacute period following epilepsy induction and may play a role in the pathological network reorganization that underlies epileptogenesis. These findings suggest a possible role for EAM in remodeling in TLE and represent a potential therapeutic target for preventing epileptogenesis.
Funding: AAN NRTS, NINDS K08