Learning and memory formation are complex processes requiring a delicate balance of organization, creation and destruction of synapses, or connections between neurons, and one team of researchers at Hopkins has discovered a new mechanism through which these processes are controlled.
The team, led by Valina and Ted Dawson and Jianmin Zhang at the Hopkins School of Medicine, characterized and named a novel AAA+ ATPase protein, Thorase, and have implicated Thorase in the deconstruction phase of synapse growth and development.
“We have been working on novel cell survival molecules and Thorase was identified in one of our screens,” Ted Dawson said. “In the process of figuring out how it was neuroprotective, we discovered that it is [a] major regulator of AMPA receptor trafficking and learning and memory.”
As an AAA+ ATPase, Thorase is most likely involved in crucial cellular processes such as protein processing and activation, quality control of intracellular macromolecules as well as maintenance of cellular structures and components. Indeed, the current study shows exactly that.
Thorase in particular has been shown to regulate the composition of the post-synaptic structure by regulating how many AMPA receptors (AMPARs) are present. AMPARs are receptors for an excitatory neurotransmitter, glutamate, which plays a crucial role in the signaling events leading to cellular memory and learning pathways.
Zhang and the Dawsons have shown in the current study that increased Thorase expression and activity leads to a decrease in AMPAR levels at the surface of post-synaptic neurons. Decreased AMPAR levels have been shown to significantly modify the post-synaptic cell’s response properties, and it is in this manner that Thorase is thought to affect learning and memory.
As AMPAR levels decline, the post-synaptic cell becomes primed to respond to signals. It shows increased amplitudes of mini excitatory post-synaptic potentials, which can be used to track a neuron’s general excitability and readiness to fire an action potential. The cell also shows enhanced long-term potentiation (LTP), which is strengthened synaptic signaling akin to cellular memory. Finally, with increased LTP and synaptic strength comes a virtual elimination of the ability to instigate long-term depression (LTD), LTP’s antagonist.
While the connection between AMPAR levels and learning is readily distinguishable, determining how Thorase affects AMPAR levels has taken a bit of work. Zhang and colleagues have approached the problem from a variety of angles; they have studied Thorase-overexpressing and Thorase-knock-out cells and have then examined changes in the levels and localizations of other proteins.
In these assays, several protein profiles were modified including GRIP-1, which is known to act as a “scaffold” in the post-synaptic density, and GluR2, a protein component of AMPARs.
“We conducted a screen for plasticity/survival proteins/genes . . . [and] began work on a clone that had not been described before. Protein-protein interaction studies suggest that Thorase bound to the scaffolding protein GRIP1 which then led to a logical series of studies,” said Valina Dawson.
From those studies, it has become apparent that Thorase interacts with GRIP-1, a glutamate-receptor interacting protein, in a way that interferes with its binding to AMPAR subunits like GluR2. When binding between GRIP-1 and GluR2 is inhibited, this complex falls apart and AMPARs are maintained at much lower levels at the post-synaptic cell surface. On the other hand, when there is too little or no Thorase around, AMPAR levels are much higher and cells can actually become overstimulated.
All of these synaptic changes contribute to the pathological phenotypes observed by Zhang and colleagues in Thorase-abnormal mice, which are unable to learn properly. The mice that lacked Thorase that were examined in the current study performed significantly worse than control mice at simple memory and learning tasks, indicating that Thorase’s activity, even though it acts in a deconstructive manner, is crucial for proper memory consolidation and learning capabilities.
Thorase’s activity depends on ATP. As ATP is a sort of “energy currency” within cells, this implies that the cell is investing time and energy into the process of deconstruction mediated by Thorase. In this way, it is even more obvious that the deconstruction of synapses is an important process in neurons for proper cellular signaling and function. Importantly, Thorase or other enzymes like it may be playing a role in neurological disorders of learning and memory such as autism, post traumatic stress disorder and even general memory dysfunction.
“The diseases that were mentioned have implicated glutamate dysfunction as a major contributor to the neurologic dysfunction,” Valina Dawson said. “We hope [to] understand [how] human disease might be regulated by this system and whether we can identify new treatment strategies.”
Ted Dawson agreed. “Ultimately we would like to design compounds that could enhance and inhibit Thorase’s function and to use them as memory enhancers in treatments of disorders like autism and memory erasers in disorders like PTSD,” he said.
Future studies will focus on other binding partners and actions of Thorase in order to more clearly elucidate the mechanisms behind Thorase-dependent deficits in learning and memory. Thorase is a particularly interesting molecular target for researchers because proper functioning of most systems is all about balance. Both too much and too little activity of any enzyme can create deficits.
“In thinking about treating human disease it would be about restoring balance to restore function,” Valina Dawson said. “Because Thorase is an ATPase, an enzyme, it is potentially a good drug target and thus it is possible that drugs could be developed to regulate Thorase function and thus the biologic actions of Thorase to restore balance.”
