Abstract
The Pten and Tsc1 genes both encode proteins that repress mechanistic target of rapamycin (mTOR) signaling. Disruption of either gene in the brain results in epilepsy and autism-like symptoms in humans and mouse models, therefore it is important to understand the molecular and physiological events that lead from gene disruption to disease phenotypes. Given the similar roles these two molecules play in the regulation of cellular growth and the overlap in the phenotypes that result from their loss, we predicted that the deletion of either the Pten or Tsc1 gene from autaptic hippocampal neurons would have similar effects on neuronal morphology and synaptic transmission. Accordingly, we found that loss of either Pten or Tsc1 caused comparable increases in soma size, dendrite length and action potential properties. However, the effects of Pten and Tsc1 loss on synaptic transmission were different. Loss of Pten lead to an increase in both excitatory and inhibitory neurotransmission, while loss of Tsc1 did not affect excitatory neurotransmission and reduced inhibitory transmission by decreasing mIPSC amplitude. Although the loss of Pten or Tsc1 both increased downstream mTORC1 signaling, phosphorylation of Akt was increased in Pten-ko and decreased in Tsc1-ko neurons, potentially accounting for the different effects on synaptic transmission. Despite the different effects at the synaptic level, our data suggest that loss of Pten or Tsc1 may both lead to an increase in the ratio of excitation to inhibition at the network level, an effect that has been proposed to underlie both epilepsy and autism.
Highlights
The evolutionarily conserved mechanistic target of rapamycin (mTOR) signaling network integrates information from growth factor receptor activity, ATP, glucose, and amino acid levels to control cellular growth, proliferation, and autophagy (Sarbassov et al, 2005a; Wullschleger et al, 2006; Laplante and Sabatini, 2012)
We verified that the cre-mediated gene deletion led to expected increases in mTOR signaling by testing for increased levels of phospho-S6 and phospho-4E binding protein (p4E-BP), as well as increased soma size, which are three well-characterized consequences of hyperactive mTOR signaling (Figure 1)
To verify that the changes in pS6 levels and soma size were due to hyperactive mTOR in both mutants, we treated the neurons with 20 nM rapamycin at DIV 7 for 72 h and repeated the pS6, p4EBP, and cell size measurements
Summary
The evolutionarily conserved mTOR signaling network integrates information from growth factor receptor activity, ATP, glucose, and amino acid levels to control cellular growth, proliferation, and autophagy (Sarbassov et al, 2005a; Wullschleger et al, 2006; Laplante and Sabatini, 2012). The protein products of these genes are similar in many ways: (1) they are both repressors of mTOR signaling and in their absence the mTORC1 complex is hyperactive, (2) their loss in neurons can lead to hyperexcitable network activity, and (3) rapamycin, an inhibitor of mTOR, blocks the cellular and behavioral consequences of their loss in animal models, and potentially humans as well (Krueger et al, 2013). This evidence predicts that the effects of Pten or Tsc1/2 disruption on neuronal form and function may be similar
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