Many roads to hyperexcitation: mTOR suppressors regulate synaptic function in different ways
Rajalaxmi Natarajan, PhD
November 17, 2014
Loss of 2 genes that repress mechanistic Target of Rapamycin (mTOR), a growth-promoting pathway, causes autism and epilepsy. Excessive excitation of neurons was assumed to be the culprit. However, a recent study published in the February 2014 issue of The Frontiers in Molecular Neuroscience1 adds a new twist.
mTOR is an intracellular signaling pathway that acts as a central regulator of growth and metabolism in various organs of the body. It senses varied signals, such as cellular energy levels or nutrient needs and responds by producing more proteins. Pten and Tsc1 genes normally suppress mTOR activity, which means loss of their function results in mTOR hyperactivity. It is believed that mTOR hyperactivity is the cause of autism, epilepsy, and intellectual disability in humans and mouse models lacking Pten or Tsc1.
Until now, it was thought that Pten and Tsc1 genes affect neurotransmission identically. That is, absence of these genes causes neuronal hyperexcitability by increasing release of excitatory neurotransmitters. But this had never been compared in identical experimental conditions.
As Matthew Weston, a postdoctoral researcher in Dr. John Swann’s lab and the lead author of this study says, “There is a large literature on both of these genes but most of the studies were not done in comparable conditions. They were done in different models using different conditions. So, we thought it would be worthwhile to compare the two models side-by-side.”
Also, earlier studies had used whole brain slices, which contain numerous complex neural circuits, influenced by many parameters, further confounding the issue.
For this reason, researchers at the Swann lab used a simple, yet powerful system to study activity of individual neurons in Pten or Tsc1 knock out (KO) mice. They grew single neurons isolated from hippocampus, brain region essential for memory and information processing, in dishes in an island of astrocytes, the support cells. Such “autaptic” cultures result in self-synapses, meaning the axon of that neuron connects to its own dendrites. This simple model bypassed the complexities of a neural network and allowed researchers to measure the activity of individual neurons.
Excitatory neurotransmitters increase the likelihood that a neuron will fire whereas inhibitory neurotransmitters do the opposite. Interestingly, loss of Pten increased excitatory as well as inhibitory neurotransmission of individual neurons, but not to the same extent. Researchers found that the gain in excitatory neurotransmission in these KO mice was likely overwhelmed by the gain in inhibitory transmission. Thus, the net effect was that Pten KO neurons were hyperexcited.
Similar analysis of Tsc1 KO neurons led to another unexpected finding. Loss of Tsc1 did not affect the release of excitatory neurotransmitters from individual neurons. If so, researchers were curious to know why these neurons were in a state of constant excitation? Also, it is known from earlier studies, that human or mice brain samples deficient for Tsc1 exhibit overall network hyperexcitability.2
The answer to this puzzling question was simple. Tsc1 mutant neurons have reduced inhibitory neurotransmission. Lack of inhibitory regulation meant that the net effect on neurons was as if they were being constantly excited. Thus, reduced inhibition of neuronal activity was enough to perturb the fine balance in the network, which eventually tilted towards hyperexcitation.
To conclude, it appears that despite some functional similarities shared by Pten and Tsc1 (repression of mTOR, similar outcomes in terms of overall network activity and disease symptoms in KO models), the way they affect the functioning of individual neurons is very different. The present study hints that this differential response may be largely due to altered numbers of postsynaptic inhibitory receptors.
Currently, researchers at the Swann lab are taking these exciting findings a step further by trying to recapitulate them in Pten and Tsc1 KO mice brains.
In addition to identifying distinct synaptic roles for two mTOR suppressors, this study has important clinical implications.
Rapamycin, an inhibitor of mTOR, is used extensively to treat many types of cancers. Clinical trials are underway to test its effectiveness in treating autism, epilepsy and cognitive dysfunction due to mTOR hyperactivity. While initial results from some of those trials are optimistic, findings from this study caution against using rapamycin as a single ‘magic bullet’ to treat all mTOR-related neurocognitive conditions.
As Weston said, “Now that we know these mTOR suppressors cause neurological disorders via distinct mechanisms, we may not want to target a general molecule like TOR to treat all these patients. We may be better off using drugs that specifically target specific mutations that underlie each disorder.”
- Weston MC, Chen H, Swann JW. Front Mol Neurosci. 2014 Feb 3;7:1. Loss of mTOR repressors Tsc1 or Pten has divergent effects on excitatory and inhibitory synaptic transmission in single hippocampal neuron cultures.
- Wang Y, Greenwood JS, Calcagnotto ME, Kirsch HE, Barbaro NM, Baraban SC. Ann Neurol. 2007 Feb; 61(2):139-52. Neocortical hyperexcitability in a human case of tuberous sclerosis complex and mice lacking neuronal expression of TSC1.