Epileptic insults profoundly alter hippocampal dentate granule cell (DGC) integration, leading to morphological and functional dendritic abnormalities as well as cell migration defects. Typically, hippocampal DGCs are generated throughout adult life from multipotent and self-renewing progenitor cells located in the subgranular zone, including Gli1-expressing type 1 progenitor cells. Immediately prior to and following epileptic insults misdirected DGCs populate the dentate hilus in addition to the dentate gyrus and afferent inputs to these “vagabond” DGCs exhibit atypical firing patterns. Dendrites, which typically develop apically during normal DGCs maturation, develop basally following epileptic seizures, often creating recurrent circuits that may contribute to hyperexcitability in the hippocampus. Although the abnormal DGCs that form following epileptic events are well characterized, questions about the specific precursors from which they arise remain unanswered. Recently, a research team led by Dr. Steve Danzer at Cincinnati Children's Hospital Medical Center and University of Cincinnati investigated whether the abnormal DGCs that are born following epileptic insults arise from all DGC precursor cells or just from a specific precursor subset (Singh et al. 2016).
To examine DGCs descended from the Gli1-expressing granule cell lineage, the Danzer team bred mice carrying a cre-conditional “Brainbow” reporter (STOCK- Gt(ROSA)26Sortm1(CAG-Brainbow2.1)Cle/J (013731) to a tamoxifen-inducible Gli1-CreERT2 mouse (STOCK-Gli1tm3(cre/ERT2)Alj/J (007913) and induced epilepsy in double mutant mice using pilocarpine. Treating double-mutants mice postnatally with tamoxifen labelled type 1 progenitor cells in the subgranular zone with one of four possible fluorescent proteins. Further, daughter cells that differentiated from a given labelled precursor cell were labelled with the same fluorescent tag as the parent. For their study, the Danzer team morphologically examined cells expressing either red fluorescent protein (RFP) or yellow fluorescent protein (YFP).
In mice, acute status epilepticus (SE) and subsequent epilepsy develop within a few weeks following pilocarpine treatment. In the Danzer study, mice hippocampi were examined 2 months post-pilocarpine treatment, when frequent spontaneous seizure activity typically is observed. Examining Brainbow-derived fluorophore expression following SE revealed that ectopic DGCs - that is, the DGCs that inappropriately migrated to the denate hilus - were concentrated within distinct cell clusters, many of which only contained other ectopic DGCs.
Interestingly, although hyperexcitable afferents and atypical connections are pathological hallmarks of ectopic DGCs, the number of DGCs with abnormal, basally-derived dendrites was not higher in ectopic DGC clusters than in normal DGC clusters. These data suggest that progenitor cells that give rise to ectopic DGCs are not more likely to produce DGCs with abnormal morphology and that the progenitor cells that produce DGCs with basal dendrites will mostly produce normal DGCs.
Taken together, these findings indicate that the mechanisms that underlie ectopic DGC development and basal dendrite formation are distinct. The Danzer team’s data suggests that combinatorial therapeutic approaches that target each of these pathways may be necessary to combat the abnormalities associated with epileptic disease.