Brain cell-specific origin of circulating microRNA biomarkers in experimental temporal lobe epilepsy

The diagnosis of epilepsy is complex and challenging and would benefit from the availability of molecular biomarkers, ideally measurable in a biofluid such as blood. Experimental and human epilepsy are associated with altered brain and blood levels of various microRNAs (miRNAs). Evidence is lacking, however, as to whether any of the circulating pool of miRNAs originates from the brain. To explore the link between circulating miRNAs and the pathophysiology of epilepsy, we first sequenced Ago2-bound miRNAs in plasma samples collected from mice subject to status epilepticus (SE) induced by intraamygdala microinjection of kainic acid. This identified time-dependent changes in plasma levels of miRNAs with known neuronal and microglial-cell origins. To explore whether the circulating miRNAs had originated from the brain, we generated mice expressing FLAG-Ago2 in neurons or microglia using tamoxifen-inducible Thy1 or Cx3cr1 promoters, respectively. FLAG immunoprecipitates from the plasma of these mice after seizures contained miRNAs, including let-7i-5p and miR-19b-3p. Taken together, these studies confirm that a portion of the circulating pool of miRNAs in experimental epilepsy originates from the brain, increasing support for miRNAs as mechanistic biomarkers of epilepsy. 8 groups: c57 mice were subjected to status epilepticus using intra-amygdala injection of kainic acid. Plasma was collected from 10 mice and pooled in equal proportions for an n of 1 for each of 4 timepoints four, eight, twenty-four hours and 2 weeks. Small RNA sequencing using the Illumina HiSeq was performed on microRNAs bound to eluted Ago2 from plasma samples. Control mice were injected with PBS in the same manner.