Skip to main content

SUMOylation and kainate receptor synaptic plasticity

Final Report Summary - SUMOKAINATE (SUMOylation and kainate receptor synaptic plasticity)

One of the most widely used neurotransmitters in the brain is acetylcholine. When released, it acts on two types of receptors: nicotinic and muscarinic. Activation of muscarinic acetylcholine receptors (mAChRs) in the brain plays a major role in learning and memory. For example, it facilitates the induction of synaptic plasticity and enhances cognitive function. However, the specific muscarinic receptor subtype involved and the critical intracellular signalling pathways engaged have remained controversial. Of the five mAChR subtypes (M1 to M5) potentially involved in cognitive enhancement, the M1 subtype has received much attention due to its ubiquitous expression in the cortex and hippocampus. For example, learning, working memory and the induction of synaptic plasticity are all impaired in animals that don't have these receptors (M1 receptor knockout mice), whereas M1 receptor specific agonists improve cognitive function in animal models.

Long-term potentiation (LTP) is a process thought to represent a substrate for memory formation in the brain. We used a recently discovered highly selective allosteric M1 receptor agonist 77-LH-28-1 to show that it facilitates LTP induced by theta burst stimulation at Schaffer collateral synapses in the hippocampus. Theta burst stimulation is used because it resembles the natural activity of hippocampal pyramidal neurons.

Our results show that LTP facilitation occurs through the interplay between M1 receptors and two additional elements: NMDA receptors (NMDAR, a subtype of glutamate receptors) and calcium-dependent potassium channels (SK channels). NMDAR opening during theta burst stimulation was enhanced by M1 receptor activation indicating this is the mechanism for LTP facilitation. M1 receptors were found to enhance NMDAR activation by inhibiting SK channels that otherwise act to hyperpolarise postsynaptic spines and inhibit NMDAR opening. Thus we describe a novel mechanism where M1 receptor activation inhibits SK channels allowing enhanced NMDAR activity leading to a facilitation of LTP induction in the hippocampus.

These results are important because, from a therapeutic point of view, this study suggests 77-LH-28-1 and similar compounds may be highly attractive as potential treatments for cognitive disorders. Currently, the only effective treatments for patients with Alzheimer's disease are cholinesterase inhibitors and memantine, but we believe that the development of new allosteric M1 receptor agonists could provide a major breakthrough in the treatment of this disease.

In the previous section, we show that mAChR activation facilitates the induction of synaptic plasticity via inhibition of SK potassium channels. However, mAChR activation affects other types of potassium channels, such as those from Kv7 family that are activated by changes in membrane potential.

As before, we used patch-clamp electrophysiological recording to show this. However, a recent advancement in our methodology is the use of dynamic clamping. This means that we are now able to emulate the activation or the inhibition of various types of channels in the cellular membrane by using a mathematical model of channel activity. We have defined such a model for Kv7 channels.

Our results show that both the pharmacological inhibition of Kv7 channels (using the inhibitor XE-991) and negating Kv7 channel conductance using dynamic clamp methodologies facilitated LTP induced by theta burst stimulation at Schaffer collateral commissural synapses. Following the bursts of action potentials during theta burst stimulation, the cellular membrane remains depolarised for a certain period of time, during which the cell is more excitable than it usually is. This phenomenon is called after-depolarisation.

As opposed to previously mentioned results, negation of Kv7 channels by XE-991 or dynamic clamp did not enhance synaptic NMDAR activation in response to theta burst synaptic stimulation. Instead, Kv7 channel inhibition increased the amplitude and duration of the after-depolarisation following a burst of action potentials. Furthermore, the effects of pharmacological blockade by XE-991 were reversed by re-introducing a Kv7-like conductance with dynamic clamp.

These data reveal that Kv7 channel inhibition promotes NMDAR opening during LTP induction by enhancing depolarisation during and after bursts of postsynaptic action potentials. Thus, this is an additional pathway, complementary to the one described above, that M1 mAChRs use to facilitate LTP.

Homeostatic synaptic scaling is a form of synaptic plasticity that, acting in a negative-feedback manner, serves to adjust the strength of excitatory synapses in order to stabilise firing. In other words, it appears that neurons are able to detect changes in their own firing rates through a set of calcium-dependent sensors. Then, they act accordingly (i.e. in the opposite direction) to regulate receptor trafficking so that there is an increase or decrease in the accumulation of glutamate receptors at synaptic sites. Importantly, as opposed to LTP, this phenomenon is slow to develop and is not synapse specific, affecting the whole neuron.

The purpose of this research was to determine the role of SUMOylation in the scaling of excitatory synaptic transmission evoked by chronic downregulation of synaptic activity (TTX was applied to prevent synaptic activity).

The results show that, in the slices treated with TTX, there was a significant decrease in AMPAR-mediated responses in the cells overexpressing SENP1, compared to non-infected, control cells. In the control group of cells (not treated with TTX), there was no difference in the amplitude of AMPAR-mediated responses between infected and non-infected cells. In addition, overexpression of SENP1 did not produce any change in rectification index, compared to non-infected cells, both in TTX-treated and control slices.

Our conclusion from these data is that SUMOylation plays an important role in synaptic scaling of synaptic responses. In addition, these data are in concordance with those obtained by molecular biology and confocal imaging.