Abnormalities of granule cell dendritic structure are a prominent feature of the intrahippocampal kainic acid model of epilepsy despite reduced postinjury neurogenesis

Department of Anesthesia, Cincinnati Children's Hospital Medical Center, 3333 Burnet Avenue, Cincinnati, OH 45229, U.S.A.
Epilepsia (Impact Factor: 4.57). 05/2012; 53(5):908-21. DOI: 10.1111/j.1528-1167.2012.03463.x
Source: PubMed


  Aberrant plastic changes among adult-generated hippocampal dentate granule cells are hypothesized to contribute to the development of temporal lobe epilepsy. Changes include formation of basal dendrites projecting into the dentate hilus. Innervation of these processes by granule cell mossy fiber axons leads to the creation of recurrent excitatory circuits within the dentate. The destabilizing effect of these recurrent circuits may contribute to hyperexcitability and seizures. Although basal dendrites have been identified in status epilepticus models of epilepsy associated with increased neurogenesis, we do not know whether similar changes are present in the intrahippocampal kainic acid model of epilepsy, which is associated with reduced neurogenesis.
  In the present study, we used Thy1-YFP-expressing transgenic mice to determine whether hippocampal dentate granule cells develop hilar-projecting basal dendrites in the intrahippocampal kainic acid model. Brain sections were examined 2 weeks after treatment. Tissue was also examined using ZnT-3 immunostaining for granule cell mossy fiber terminals to assess recurrent connectivity. Adult neurogenesis was assessed using the proliferative marker Ki-67 and the immature granule cell marker calretinin.
  Significant numbers of cells with basal dendrites were found in this model, but their structure was distinct from basal dendrites seen in other epilepsy models, often ending in complex tufts of short branches and spines. Even more unusual, a subset of cells with basal dendrites had an inverted appearance; they completely lacked apical dendrites. Spines on basal dendrites were found to be apposed to ZnT-3 immunoreactive puncta, suggestive of recurrent mossy fiber input. Finally, YFP-expressing abnormal granule cells did not colocalize Ki-67 or calretinin, indicating that these cells were more than a few weeks old, but were found almost exclusively in proximity to the neurogenic subgranular zone, where the youngest granule cells are located.
  Recent studies have demonstrated in other models of epilepsy that dentate pathology develops following the aberrant integration of immature, adult-generated granule cells. Given these findings, one might predict that the intrahippocampal kainic acid model of epilepsy, which is associated with a dramatic reduction in adult neurogenesis, would not exhibit these changes. Herein we demonstrate that hilar basal dendrites are a common feature of this model, with the abnormal cells likely resulting from the disruption of juvenile granule cell born in the weeks before the insult. These studies demonstrate that postinjury neurogenesis is not required for the accumulation of large numbers of abnormal granule cells.

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    • "Subsequent studies revealed that these sprouted axons form excitatory synaptic connections with granule cell dendrites projecting through the IML, creating recurrent loops which have been hypothesized to promote hyperexcitability (for review, see Nadler, 2003). More recently, studies using models of temporal lobe epilepsy have identified ectopic granule cells (Parent et al., 1997; Scharfman et al., 2000; Overstreet-Wadiche et al., 2006), hypertrophied granule cells (Suzuki et al., 1995; Murphy et al., 2011, 2012), granule cells with basal dendrites (Ribak et al., 2000, 2012; Overstreet-Wadiche et al., 2006; Murphy and Danzer, 2011), and granule cells with altered synaptic structure (Pierce and Milner, 2001; Danzer et al., 2010; McAuliffe et al., 2011; Upreti et al., 2012) as common pathologies of the disorder. Interestingly, the dentate gyrus is one of only two regions exhibiting persistent neurogenesis in adulthood, and a majority of the granule cells exhibiting these pathological abnormalities appear to be newborn (Parent et al., 2006; Walter et al., 2007; Kron et al., 2010; Santos et al., 2011). "
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    ABSTRACT: The phosphatidylinositol-3-kinase/phosphatase and tensin homolog (PTEN)-mammalian target of rapamycin (mTOR) pathway regulates a variety of neuronal functions, including cell proliferation, survival, growth, and plasticity. Dysregulation of the pathway is implicated in the development of both genetic and acquired epilepsies. Indeed, several causal mutations have been identified in patients with epilepsy, the most prominent of these being mutations in PTEN and tuberous sclerosis complexes 1 and 2 (TSC1, TSC2). These genes act as negative regulators of mTOR signaling, and mutations lead to hyperactivation of the pathway. Animal models deleting PTEN, TSC1, and TSC2 consistently produce epilepsy phenotypes, demonstrating that increased mTOR signaling can provoke neuronal hyperexcitability. Given the broad range of changes induced by altered mTOR signaling, however, the mechanisms underlying seizure development in these animals remain uncertain. In transgenic mice, cell populations with hyperactive mTOR have many structural abnormalities that support recurrent circuit formation, including somatic and dendritic hypertrophy, aberrant basal dendrites, and enlargement of axon tracts. At the functional level, mTOR hyperactivation is commonly, but not always, associated with enhanced synaptic transmission and plasticity. Moreover, these populations of abnormal neurons can affect the larger network, inducing secondary changes that may explain paradoxical findings reported between cell and network functioning in different models or at different developmental time points. Here, we review the animal literature examining the link between mTOR hyperactivation and epileptogenesis, emphasizing the impact of enhanced mTOR signaling on neuronal form and function.
    Frontiers in Molecular Neuroscience 03/2014; 7:18. DOI:10.3389/fnmol.2014.00018 · 4.08 Impact Factor
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    • "Unilateral injection of kainate (KA), an agonist of the excitatory neurotransmitter glutamate, into the hippocampus of adult mice not only induced epileptic GC activity and epileptic seizures, but also GCD, on the side of KA injection but not on the contralateral side (Bouilleret et al. 1999; Heinrich et al. 2006). It has been shown previously that postnatal neurogenesis of dentate GCs is abolished near the site of KA injection where GCD is maximal (Heinrich et al. 2006; Nitta et al. 2008; Murphy et al. 2012; Häussler et al. 2012; Sibbe et al., 2012), making it unlikely that GCD develops by the aberrant migration of adult-generated GCs. Does GCD in epilepsy result from migratory activity of fully differentiated GCs despite that they are fixed in place by thousands of synapses? "
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    ABSTRACT: Neuronal ectopia, such as granule cell dispersion (GCD) in temporal lobe epilepsy (TLE), has been assumed to result from a migration defect during development. Indeed, recent studies reported that aberrant migration of neonatal-generated dentate granule cells (GCs) increased the risk to develop epilepsy later in life. On the contrary, in the present study, we show that fully differentiated GCs become motile following the induction of epileptiform activity, resulting in GCD. Hippocampal slice cultures from transgenic mice expressing green fluorescent protein in differentiated, but not in newly generated GCs, were incubated with the glutamate receptor agonist kainate (KA), which induced GC burst activity and GCD. Using real-time microscopy, we observed that KA-exposed, differentiated GCs translocated their cell bodies and changed their dendritic organization. As found in human TLE, KA application was associated with decreased expression of the extracellular matrix protein Reelin, particularly in hilar interneurons. Together these findings suggest that KA-induced motility of differentiated GCs contributes to the development of GCD and establish slice cultures as a model to study neuronal changes induced by epileptiform activity.
    Cerebral Cortex 03/2013; 24(8). DOI:10.1093/cercor/bht067 · 8.67 Impact Factor
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    ABSTRACT: The dentate gyrus is hypothesized to function as a "gate," limiting the flow of excitation through the hippocampus. During epileptogenesis, adult-generated granule cells (DGCs) form aberrant neuronal connections with neighboring DGCs, disrupting the dentate gate. Hyperactivation of the mTOR signaling pathway is implicated in driving this aberrant circuit formation. While the presence of abnormal DGCs in epilepsy has been known for decades, direct evidence linking abnormal DGCs to seizures has been lacking. Here, we isolate the effects of abnormal DGCs using a transgenic mouse model to selectively delete PTEN from postnatally generated DGCs. PTEN deletion led to hyperactivation of the mTOR pathway, producing abnormal DGCs morphologically similar to those in epilepsy. Strikingly, animals in which PTEN was deleted from ≥ 9% of the DGC population developed spontaneous seizures in about 4 weeks, confirming that abnormal DGCs, which are present in both animals and humans with epilepsy, are capable of causing the disease.
    Neuron 09/2012; 75(6):1022-34. DOI:10.1016/j.neuron.2012.08.002 · 15.05 Impact Factor
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