Seizure-Induced Motility of Differentiated Dentate Granule Cells Is Prevented by the Central Reelin Fragment

Abstract

Granule cell dispersion (GCD) represents a pathological widening of the granule cell layer in the dentate gyrus and it is frequently observed in patients with mesial temporal lobe epilepsy (MTLE). Recent studies in human MTLE specimens and in animal epilepsy models have shown that a decreased expression and functional inactivation of the extracellular matrix protein Reelin correlates with GCD formation, but causal evidence is still lacking. Here, we used unilateral kainate (KA) injection into the mouse hippocampus, an established MTLE animal model, to precisely map the loss of reelin mRNA-synthesizing neurons in relation to GCD along the septotemporal axis of the epileptic hippocampus. We show that reelin mRNA-producing neurons are mainly lost in the hilus and that this loss precisely correlates with the occurrence of GCD. To monitor GCD formation in real time, we used organotypic hippocampal slice cultures (OHSCs) prepared from mice which express enhanced green fluorescent protein (eGFP) primarily in differentiated dentate granule cells. Using life cell microscopy we observed that increasing doses of KA resulted in an enhanced motility of eGFP-positive granule cells. Moreover, KA treatment of OHSC resulted in a rapid loss of Reelin-producing interneurons mainly in the hilus, as observed in vivo. A detailed analysis of the migration behavior of individual eGFP-positive granule cells revealed that the majority of these neurons actively migrate toward the hilar region, where Reelin-producing neurons are lost. Treatment with KA and subsequent addition of the recombinant R3–6 Reelin fragment significantly prevented the movement of eGFP-positive granule cells. Together, these findings suggest that GCD formation is indeed triggered by a loss of Reelin in hilar interneurons.

Reelin: Neurodevelopmental Architect and Homeostatic Regulator of Excitatory Synapses

“…Inhibition of TIMP1 restored the Nt cleavage of Reelin and reversed the KA-induced GCD (43). Furthermore, perfusion of the central fragment of Reelin could prevent this GCD (44). Although these studies demonstrate that GCD results from both the loss of Reelin-positive neurons and hindered proteolysis of Reelin after epileptic insult, the question still remains whether the recurrence of seizures in TLEs is due to GCD.…”

“…Later studies were able to recapitulate the KA-induced GCD in rodent organotypic hippocampal slice cultures, which resulted in a rapid loss of Reelin-expressing hilar neurons. These organotypic hippocampal slice cultures also had elevated expression of the matrix metalloprotease (MMP) inhibitor, tissue inhibitor of MMP-1 (TIMP1), which led to reduced Nt cleavage and diffusion of Reelin (42)(43)(44). Inhibition of TIMP1 restored the Nt cleavage of Reelin and reversed the KA-induced GCD (43).…”

Hippocampal granule cell dispersion: a non-specific finding in pediatric patients with no history of seizures

“…Biochemical and functional studies in model systems confirm that formation of a densely packed GC layer is dependent on proper Reelin signaling [58,62]. Further, human studies as well as genetic and chemical-induced mouse models of epilepsy showed association between reduced Reelin mRNA expression and GCD [13,16,22,24,27,42]; although there is discrepancy in the type of Reelin + cells (early-born Cajal-Retzius cells or late-born hilar interneurons) considered to be associated with GCD. Introduction of exogenous Reelin into kainate-injected mouse hippocampus was also reported to prevent GCD in mice [41].…”

Hippocampal morphometry in sudden and unexpected death in epilepsy (SUDEP)

“…Granule cell dispersion is commonly associated with HS in TLE, noted in 76% of surgical specimens in a recent series 17 . In the context of HS, seizure-induced loss of reelinsynthesising interneurons has been shown to initiate neo-migration of mature granule cells 18 .…”