Role of Glutamate and NMDA Receptors in Alzheimer’s Disease

NMDAR-mediated glutamatergic signaling in synaptic plasticity and neuronal survival

One subgroup of iGluRs is selectively gated by specific agonists N-methyl-d-aspartate (NMDA), thus named as NMDA receptor (NMDAR) [12]. NMDAR is distinct than other iGluRs in its voltage-dependent activation via removal of Mg²⁺ blockade, its high Ca²⁺ permeability, and relatively slow ligand-gated kinetics. These unique features render NMDAR unique and essential for its crucial role in synaptic function and plasticity [13, 14]. Basically, at a resting membrane potential of about –70 mV, the Ca²⁺ channel of NMDAR is blocked by Mg²⁺. During the induction of long-term potentiation, however, the strong and prolonged release of glutamate from the presynaptic terminal activates AMPARs and the subsequent depolarization removes the Mg²⁺ blockade of the NMDAR channel and allows the influx of Ca²⁺ ion. This strong activation of NMDARstriggers a Ca2+/calmodulin-dependent protein kinase II (CaMKII)-mediated signaling cascade that eventually leads to an enhanced synaptic strength. On the contrary, a modest activation of NMDARs causes a modest increase in postsynaptic Ca²⁺ and triggers phosphatase-mediated long-term depression [15].

In addition to their crucial roles in synaptic transmission and plasticity, NMDARs seem to be critical for the survival of neurons by activating neuronal survival pathway [16, 17]. To support this, studies indicate that the blockade of NMDAR function leads to neuronal apoptosis and degeneration [18, 19]. This NMDAR-dependent neuroprotective function mainly involves the activation of pro-survival transcription factors and the inhibition of apoptosis [20, 21]. The activation of synaptic NMDAR promotes survival gene expression by activating Ca²⁺-dependent transcription factors such as cyclic-AMP response element binding protein (CREB) and suppresses caspases and apoptotic pathway such as Puma (pro-apoptotic Bcl2 homology domain 3 (BH3)-only member gene)-activated signaling cascade and transcription factor FOXO (forkhead box protein O)-induced pro-death gene expression [20, 21]. The NMDAR-dependent synaptic plasticity and activation of cell survival signaling is illustrated in Fig. 2.

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Fig.2

Activation of synaptic NMDAR mediates survival pathway and induces long-term potentiation (LTP) and long-term depression (LTD). In classical synaptic plasticity model, synaptic NMDAR activity activates CaMKII and phosphatases, which triggers LTP and LTD, respectively. Moreover, the activation of synaptic NMDARs suppresses the pro-apoptotic signaling molecules and pathways such as caspases, APAF1 and Puma in cytoplasm; and it also promotes the expression of the pro-survival transcriptional factors such as CREB and suppresses the expression of pro-apoptotic transcription factors such as FOXO and p53, which, in turn, inhibits apoptosis and promotes cell survival.

Glutamate excitotoxicity and abnormal NMDAR activity in Alzheimer’s disease

Insufficient synaptic NMDAR signaling compromises neuronal cell survival. Excessive stimulation of glutamatergic signaling, however, results inexcitotoxicity, in which nerve cells are damaged or killed, or neurological trauma such as stroke [22]. Besides acute effects, many studies indicate a role for glutamate excitotoxicity in delayed slowly-evolving neurodegeneration [23, 24]. Accumulating evidence demonstrates that the toxicity is principally mediated by excessive Ca²⁺ entry, primarily through NMDARs [25–28], since NMDARs have a much higher permeability for calcium ions compared to other iGluRs [29]. In this regard, the modest depolarization of the postsynaptic membrane and other factors that relieve the Mg²⁺ blockade can activate NMDARs in a mild and chronic way, which causes the prolonged Ca²⁺ influx into the postsynaptic neuron. The pathological level of Ca²⁺ signaling leads to gradual loss of synaptic function and ultimate neuronal cell death, which correlates clinically with the progressive decline in cognition/memory and the development of pathological neural anatomy seen in AD patients, and this, in turn, rationalizes the clinical trial of memantine, an NMDAR antagonist, as a symptomatological and neuroprotective treatment for AD [30–32]. Thus, given the fact that NMDARs are also important for cell survival, the level of NMDAR signaling must be maintained at a proper level so that it is enough to promote neuronal survival but not harmful to cause neurodegeneration as occurred in AD. The major factors that affect NMDAR signaling in AD include glutamate availability and the modulation of NMDAR channel functions.

Glutamate uptake and recycling system is an important factor that determines the availability of glutamate for signaling processes. Unfortunately, in AD, this system can be severely weakened. In AD patients, a decrease in the glutamate transporter capacity and protein expression and a selective loss of vesicular glutamate transporter (VGluT) were seen [33–35]. Moreover, excitatory amino acid transporter 2 (EAAT2), which is primarily located in perisynaptic astrocytes, was reported to have impaired function in AD [36]. Studies using various species of Aβ peptides in neuronal cell culture seem to support the same idea that toxic Aβ may allow more glutamate availability by impairing glutamate uptake/recycling mechanisms [37–39]. This enhanced glutamate supply is likely to contribute to AD-associated excitotoxicity and neurodegeneration.

The integrity of the presynaptic neurotransmitter release machinery may also contribute to glutamate availability. It was reported that Aβ can significantly reduce the expression of presynaptic protein such as synaptophysin, syntaxin, and synaptotagmin, many of which are active components of the neurotransmitter release machinery [40]. Other research pointed out that endogenous Aβ played a key part in the regulation of activity-dependent synaptic vesicle release [41]. The deficits in the presynaptic vesicle release machinery supposedly compromises glutamate availability, making it less possible to initiate an excitotoxicity effect. However, it is consistent with the pathological synaptic loss observed in AD and it is likely to be a relatively later effect occurring in ongoing degenerating neurons.

In addition to the elevated level of glutamate, AD may enhance NMDAR signaling through the modulation of the receptor itself.

Numerous studies have demonstrated that Aβ directly modulates the electrophysiological function of NMDARs. In general, species of Aβ causes elevated NMDAR-mediated synaptic currents and collateral toxicity, which can be attenuated or blocked by NMDAR antagonists, such as MK-801 [42–44], D-APV, or memantine [45–48]. The structural effects of Aβ, such as synaptic loss, can also be prevented by NMDAR antagonists [49, 50]. Aβ may even physically interact with NMDARs, either directly or via synaptic proteins such as PSD95 [51–53].

AD may also affect the level of NMDAR coagonists. The complete activation of NMDARs by glutamate requires the continuous binding of coagonist D-serine or glycine, and therefore these coagonists play important modulatory role in NMDAR function. In AD hippocampus or Aβ-treated cultured microglia, both D-serine and the expression of serine racemase, which generates D-serine, are reported to beincreased [54]. Furthermore, the knockout of serine racemase, which significantly reduce the forebrain D-serine content, ameliorated the NMDA or Aβ caused neurotoxicity [55].

Taken together, it is widely accepted that Aβ-induced changes in the availability of glutamate and the function of NMDAR channels correlate with the neurotoxicity and degeneration observed in AD.

Regionalized NMDAR signaling and Alzheimer’s disease

It seems that Ca²⁺ level via NMDAR signaling is critical in determining cell fate; insufficient signaling leads to failure in cell survival, while too much signaling causes excitotoxicity and neurodegeneration. However, emerging evidence indicates that this is only part of the story. The membranous location and regionalized signaling of NMDARs may be key to AD-associated pathophysiology.

The NMDARs on membrane can be roughly divided into two population groups: synaptic and extrasynaptic. Compared to numerous studies conducted with synaptic NMDARs, few were done to extrasynaptic NMDARs and their function remains largely unknown until recently. A few recent studies contend that extrasynaptic NMDARs may be involved in or responsible for glutamate excitotoxicity and cell death [56–59]. Although there are still opposing reports, a widely accepted model is that synaptic NMDAR activation promotes cell survival whereas extrasynaptic activation triggers cell death and the tilted balance between synaptic and extrasynaptic NMDAR activity contributes to neuronal dysfunction such as acute cell trauma and chronic neurodegenerative diseases [21]. Moreover, synaptic and extrasynaptic NMDARs were found to be activated by different endogenous coagonists, D-serine, and glycine, respectively [60]. Interestingly, memantine, an NMDAR antagonist, was reported to target preferentially against extrasynaptic NMDAR [58].

Extrasynaptic NMDAR-induced responses seem to be tightly related to the physiological changes occurred in AD. A recent study demonstrated that Aβ specifically activated extrasynaptic NMDARs, which caused synaptic loss, and memantine antagonized Aβ induced negative effects [61]. This regionalized NMDAR signaling model can also be used to address Aβ-caused decrease in NMDAR expression [62–64]. The decreased number of NMDARs may simply disrupt the balance between synaptic and extrasynaptic NMDARs.

What occurs to the synaptic portion of NMDARs in AD appears to be well established. Numerous studies have shown that soluble Aβ causes the reduction of synaptic glutamatergic transmission and the inhibition of synaptic plasticity. For example, one study demonstrated that application of Aβ_1–42 in cultured cortical neurons leads to the internalization of synaptic NMDARs and the depression of NMDAR-mediated currents [65]. Generally speaking, AD-associated synaptic damage or weakening of synaptic function is one of the universal themes of AD pathophysiology.

In AD, extrasynaptic NMDAR-mediated signaling pathway antagonizes the cell survival pathway mediated through synaptic NMDARs by inactivating CREB and activating FOXO transcription factors and promoting the related pro-death and oxidative stress signaling [20, 21]. Moreover, multiple mechanisms may further undermine the function of synaptic NMDAR function. As a result, the balance of the regionalized NMDAR signaling between synaptic and extrasynaptic regions is tilted towards the downstream signaling pathways that eventually lead to the death of neurons in AD, as illustrated in Fig. 3.

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Fig.3

Regionalized NMDAR activity determines cell fate. In principle, extrasynaptic NMDAR-mediated apoptosis antagonizes synaptic NMDAR-mediated survival. In Alzheimer’s disease, Aβ-induced glutamate release from astrocytes activates extrasynaptic NMDARs and triggers pro-apoptotic signaling (red), which overcomes synaptic NMDAR-mediated survival signaling (green) that is undermined by other mechanisms such as the endocytosis of NMDARs, leading to further synaptic damage and eventualneuronal death.