Axonal plasticity of age-defined dentate granule cells in a rat model of mesial temporal lobe epilepsy

Abstract

Dentate granule cell (DGC) mossy fiber sprouting (MFS) in mesial temporal lobe epilepsy (mTLE) is thought to underlie the creation of aberrant circuitry which promotes the generation or spread of spontaneous seizure activity. Understanding the extent to which populations of DGCs participate in this circuitry could help determine how it develops and potentially identify therapeutic targets for regulating aberrant network activity. In this study, we investigated how DGC birthdate influences participation in MFS and other aspects of axonal plasticity using the rat pilocarpine-induced status epilepticus (SE) model of mTLE. We injected a retrovirus (RV) carrying a synaptophysin-yellow fluorescent protein (syp-YFP) fusion construct to birthdate DGCs and brightly label their axon terminals, and compared DGCs born during the neonatal period with those generated in adulthood. We found that both neonatal and adult-born DGC populations participate, to a similar extent, in SE-induced MFS within the dentate gyrus inner molecular layer (IML). SE did not alter hilar MF bouton density compared to sham-treated controls, but adult-born DGC bouton density was greater in the IML than in the hilus after SE. Interestingly, we also observed MF axonal reorganization in area CA2 in epileptic rats, and these changes arose from DGCs generated both neonatally and in adulthood. These data indicate that both neonatal and adult-generated DGCs contribute to axonal reorganization in the rat pilocarpine mTLE model, and indicate a more complex relationship between DGC age and participation in seizure-related plasticity than was previously thought.

Rabies tracing of birthdated dentate granule cells in rat temporal lobe epilepsy

“…5,23 We recently found that both early- and adult-born DGCs contribute to mossy fiber sprouting, 17 but it remains unclear whether these populations differ in the extent to which they receive mossy fiber innervation. We used the DGC marker Prox1 to quantify mCh+/Prox1+ (GFP-negative) cells, representing DGCs that monosynaptically innervate birthdated starter neurons.…”

“…15,17 Briefly, P7 pups were anesthetized on ice and placed on a neonatal rat stereotaxic adapter (Stoelting, Wood Dale, IL) in a Kopf stereotaxic frame (Tujunga, CA). Bilateral burr holes were drilled and 1 μL of RV-Syn-GTR was injected (0.1 μL/min) using a 5 μL Hamilton syringe with the following coordinates (in mm from bregma and mm below the skull): caudal 2.0, lateral 1.5, depth 2.7.…”

“…13–16 However, recent work indicates that pre-existing DGCs also contribute to mossy fiber sprouting. 17 …”

Extended Interneuronal Network of the Dentate Gyrus

“…Avian envelope glycoprotein subgroup A (EnvA)-pseudotyped RbV (RbV-mCh) was produced as described previously (Wickersham et al, 2010) with titers of 1–8 × 10 8 cfu/mL. We injected Syn-GTR bilaterally into the dorsal dentate gyrus at postnatal day (P) 7 as previously described (Althaus et al, 2016; Kron et al, 2010) to birthdate neonatally-born dentate granule cells and render them RbV-competent. For additional details, see Supplemental Information.…”

PTEN deletion increases hippocampal granule cell excitability in male and female mice

“…During the development of temporal lobe epilepsy, adult-generated dentate granule cells integrate abnormally into the hippocampus (Shapiro et al, 2007; Walter et al, 2007; Jessberger and Parent, 2015; Singh et al, 2016; Althaus et al, 2016). Abnormal granule cells are hypothesized to mediate epileptogenesis by impairing the “dentate gate”, a property of the hippocampal circuit that normally limits excess excitation (Krook-Magnuson et al, 2015; Scharfman and Bernstein, 2015).…”