Synaptic connections undergo continuous remodeling in response to neuronal activity. This process, known as synaptic plasticity, is widely thought to underlie learning and memory.1–4
The two best-characterized forms of synaptic plasticity are long-term potentiation (LTP) and long-term depression (LTD). Both are accompanied by long-lasting changes in synaptic strength. It is widely accepted that changes in the number of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) at the synapses primarily contribute to the expression of LTP and LTD. For example, LTP is mediated by an increase in the number of AMPARs present at synapses, in various brain structures,5–12
whereas LTD is accompanied by the removal of AMPARs from synapses in hippocampus13–16
Thus, the change in synaptic strength is due, at least in part, to an increase (LTP) or decrease (LTD) in synaptic AMPARs.
At excitatory synapses in CA1 hippocampal neurons, LTP and LTD share a common pathway for their induction. Both require the activation of postsynaptic N-methyl-D-aspartate receptors (NMDARs) and an increase in Ca2+
influx. LTP and LTD are tightly regulated, where the level of Ca2+
/calmodulin (CaM) affects the activation of enzymes that are critical for controlling the balance of synaptic plasticity. Two such enzymes are Ca2+
/CaM-dependent protein phosphatase calcineurin and Ca2+
/CaMdependent protein kinase II (CaMKII). At high Ca2+
/CaM concentrations, CaMKII is activated resulting in the insertion of AMPARs and the expression of LTP.19–22
On the other hand, low Ca2+
/CaM concentration activates calcineurin, resulting in a weakening of synaptic strength and the expression of LTD.23–26
Thus, the availability of CaM can ultimately regulate the balance between CaMKII and calcineurin activation, and therefore, can play an essential role in the bidirectional control of synaptic plasticity.
Calmodulin-mediated signaling has been studied extensively in a wide range of cellular processes. It is now appreciated that calmodulin-binding proteins buffer CaM, keeping its free intracellular concentration minimal. One of the most abundant postsynaptic calmodulin-binding proteins is neurogranin (Ng), a neuron-specific protein enriched in the hippocampus and cortex.27–29
Unlike most other calmodulin-binding proteins (e.g., CaMKII, calcineurin), Ng binds to CaM only in the absence of Ca2+
. This property suggests that Ng could act as a reservoir for CaM and a sensor for Ca2+
. Thus, Ng can control CaM availability during synaptic plasticity induction, where the levels of Ca2+
are elevated. Therefore, it is not unreasonable to hypothesize that Ng may control the balance in the synaptic activation of CaMKII and calcineurin, which, in turn, affect the balance between LTP and LTD.
Our study focused on the effects of Ng on synaptic transmission and LTP induction. We showed that Ng overexpression enhanced synaptic strength by increasing AMPAR-mediated responses in CA1 neurons.30
This potentiation was due to an increased number of AMPARs at the synapses, which was dependent on neuronal activity and NMDAR activation. To determine whether Ng-mediated potentiation was dependent on Ng-CaM binding, we expressed Ng mutants that were either incapable of binding CaM (Ng-IQless or Ng-S36D) or a mutant that constitutively binds to CaM (Ng-S36AF37W), even in the presence of Ca2+
. Overexpression of any of these mutants was unable to enhance synaptic transmission, indicating that the ability of Ng to bind to and release CaM upon demand is responsible for the Ng-mediated potentiation of synaptic transmission. We have also demonstrated that Ng-mediated AMPAR insertion requires CaMKII activation and that Ng overexpression resulted in an increased activity of CaMKII at the synapse.
To assess the role of Ng in LTP induction, we induced LTP in single neurons by pairing presynaptic stimulation with postsynaptic depolarization. We found that Ng-mediated potentiation was able to occlude LTP induction, suggesting that both share a common mechanism. To test whether Ng is required for LTP induction, we used RNA interference (RNAi) to knock down the endogenous Ng. Knocking down Ng blocked LTP, confirming its importance in LTP induction.31
Taken together, these results show that Ng is required for LTP and that it has a critical role in hippocampal synaptic function and synaptic plasticity.
The balance between the induction of LTP and LTD allows neurons to maintain their ability to undergo further synaptic remodeling. Since LTP and LTD are thought to underlie learning and memory, it is no surprise then that many learning and memory deficits are associated with impaired synaptic plasticity balance. For instance, aged rats with spatial memory deficits exhibit a lower threshold for induction of LTD32,33
and a higher threshold for induction of LTP.34–36
Interestingly, there is evidence suggesting that age-related changes in LTP and LTD are attributed, at least partially, to changes in Ca2+
/CaM-mediated signaling. For example, aging is associated with an increase in calcineurin activity37
and a decrease in CaMKII activity.38
The imbalanced activation of CaMKII and calcineurin could be due, in part, to changes in the sensitivity to detect intraspine Ca2+
changes or in the availability or distribution of CaM. As mentioned above, Ng can work as a sensor for Ca2+
and a reservoir for CaM, controlling its availability and distribution. Thus, changes in Ng levels can ultimately influence the balance between CaMKII and calcineurin activation and hence the synaptic plasticity balance.
Knocking Ng out resulted in the impairment of synaptic plasticity balance as well as in learning and memory deficits.31,39
In addition, Huang et al. has also shown that Ng levels in the hippocampus are highly correlated with performance in spatial memory tasks.31
Interestingly, Ng expression decreases with aging.40
It is intriguing to speculate about the role of Ng in aging-induced synaptic plasticity deficits. For example, if a decrease in Ng level is correlated with aging-induced learning and memory deficits, then such deficits could be alleviated or reversed by increasing Ng levels in hippocampal neurons. In support of this hypothesis, agents that increase Ng levels (e.g., vitamin A) have been shown to partially alleviate aging-induced memory and synaptic plasticity deficits.41
Future studies are needed to investigate the possible role of Ng in synaptic plasticity deficits, as well as learning and memory deficits that are associated with aging and neurological disorders such as Alzheimer’s disease.