PTEN deletion from adult-generated dentate granule cells disrupts granule cell mossy fiber axon structure

Abstract

Dysregulation of the mTOR-signaling pathway is implicated in the development of temporal lobe epilepsy. In mice, deletion of PTEN from hippocampal dentate granule cells leads to mTOR hyperactivation and promotes the rapid onset of spontaneous seizures. The mechanism by which these abnormal cells initiate epileptogenesis, however, is unclear. PTEN-knockout granule cells develop abnormally, exhibiting morphological features indicative of increased excitatory input. If these cells are directly responsible for seizure genesis, it follows that they should also possess increased output. To test this prediction, dentate granule cell axon morphology was quantified in control and PTEN-knockout mice. Unexpectedly, PTEN deletion increased giant mossy fiber bouton spacing along the axon length, suggesting reduced innervation of CA3. Increased width of the mossy fiber axon pathway in stratum lucidum, however, which likely reflects an unusual increase in mossy fiber axon collateralization in this region, offset the reduction in boutons per axon length. These morphological changes predicts a net increase in granule cell >> CA3 innervation. Increased diameter of axons from PTEN-knockout cells would further enhance granule cell >> CA3 communication. Altogether, these findings suggest that amplified information flow through the hippocampal circuit contributes to seizure occurrence in the PTEN-knockout mouse model of temporal lobe epilepsy.

Impact of rapamycin on status epilepticus induced hippocampal pathology and weight gain

“…This sprouting creates recurrent excitatory circuits in the dentate gyrus, promoting local hyperexcitability (Sutula and Dudek, 2007). Mossy fiber sprouting occurs following gene mutations that enhance mTOR signaling (LaSarge and Danzer, 2014; LaSarge et al, 2015). Inhibiting the pathway with rapamycin has consistently been demonstrated to block sprouting in acquired epilepsy models (Zeng et al, 2009; Buckmaster et al, 2009).…”

Disrupted hippocampal network physiology following PTEN deletion from newborn dentate granule cells

“…PTEN KO cells contribute to mossy fiber sprouting (Pun et al, 2012), expand their axonal projections in the mossy fiber pathway (LaSarge et al, 2015), develop hilar-projecting basal dendrites (Pun et al, 2012), and occasionally migrate to ectopic locations in the dentate hilus. All of these changes have been hypothesized to contribute to recurrent circuitry in the epileptic temporal lobe.…”

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

“…After staining, sections were dehydrated in serial alcohol washes, cleared in xylenes, and hard mounted with Krystalon (EMD Millipore, Cat# 64969). Use of PTEN/Nissl co-staining was validated against our previous protocol using PTEN/NeuN to identify knockout granule cells (Pun et al, 2012; LaSarge et al, 2015; 2016), and found to be equally effective (data not shown).…”

“…To further test the hypothesis, we deleted the mechanistic target of rapamycin (mTOR) pathway inhibitor phosphatase and tensin homologue (PTEN) from a subset of postnatally-generated granule cells. PTEN deletion from granule cells produces axonal and dendritic abnormalities reminiscent of those seen in epilepsy (Kwon et al, 2006; Murphy et al, 2012; LaSarge and Danzer, 2014; LaSarge et al, 2015) and leads to the development of spontaneous seizures; demonstrating that abnormal granule cells can cause epilepsy (Pun et al, 2012). …”