In this study, YM Lu and colleagues have unambiguously demonstrated the significance of S1232 phosphorylation of the NMDA receptor subunit NR2A by the p25/Cdk5 kinase in ischemic-induced CA1 neuronal death. They elegantly show that inhibition of p25/Cdk5 by expression of dominant-negative mutants of Cdk5 protects CA1 neurons from ischemic injury. They also highlight the role of S1232 phosphorylation in this process with experimental data showing that expression of the mutant NR2A harboring alanine1232 fully protects neurons from ischemic injury. It should be noted that this study as well as a previous report from H Pant's laboratory (Li et al., 2001) suggest that S1232 phosphorylation upregulates NMDA receptor channel activity. Interestingly, p25 accumulation and Cdk5 activation following ischemia seem to occur upstream of the modification in NMDA receptor function; this is because MK-801, a specific noncompetitive antagonist of NMDA receptors, prevents neuronal...  Read more

In this study, YM Lu and colleagues have unambiguously demonstrated the significance of S1232 phosphorylation of the NMDA receptor subunit NR2A by the p25/Cdk5 kinase in ischemic-induced CA1 neuronal death. They elegantly show that inhibition of p25/Cdk5 by expression of dominant-negative mutants of Cdk5 protects CA1 neurons from ischemic injury. They also highlight the role of S1232 phosphorylation in this process with experimental data showing that expression of the mutant NR2A harboring alanine1232 fully protects neurons from ischemic injury. It should be noted that this study as well as a previous report from H Pant's laboratory (Li et al., 2001) suggest that S1232 phosphorylation upregulates NMDA receptor channel activity. Interestingly, p25 accumulation and Cdk5 activation following ischemia seem to occur upstream of the modification in NMDA receptor function; this is because MK-801, a specific noncompetitive antagonist of NMDA receptors, prevents neuronal death but does not affect p25 accumulation and Cdk5 activation. As such, what is the mechanism accountable for Ca++ entry that leads to calpain activation and p35 to p25 conversion? Lu and colleagues suggest that AMPA receptor channels are likely to be involved in the ischemic response, as systemic administration of the AMPA receptor antagonist NBQX prior to insult inhibits p25 accumulation and Cdk5 activation, and this ultimately exerts a protective effect on CA1 neuronal death. Taken together, this study offers a signaling cascade underlying ischemic brain injury that goes like this:

Ischemic injuries --> AMPA receptors activation --> Ca++ entry --> Calpain activation --> p25 accumulation --> Aberrant Cdk5 activation --> Elevated NR2A S1232 phosphorylation --> Sustained increase in NMDA receptor activity --> Neuronal death

Recently, a p25 transgenic mouse model was reported that expresses very low levels of p25 (about 30 percent of endogenous p35) in the hippocampus Angelo et al., 2003). Interestingly, these animals exhibited improved reversal learning and altered fear conditioning. Perhaps this phenotype reflects a slightly increased S1232 phosphorylation of NR2A and thus a sustained increase in NMDAR activity in the hippocampus of these transgenic mice.

Several questions remain to be answered. Is the NR2A subunit the only relevant target of p25/Cdk5 responsible for ischemia-induced neuronal death? Furthermore, how does phosphorylation of S1232 induce neuronal death? While it is clear that much effort is needed to elucidate these questions, this study significantly advances our current understanding of the mechanism underlying ischemic brain injury. In addition, it suggests that inhibition of S1232 phosphorylation may serve as a valid method to prevent neuronal death following ischemia.

View all comments by Li-Huei Tsai

Wang et al. suggest that ischemia-induced neuronal cell death may be mediated by activation of Cdk5. The authors propose that Cdk5 phosphorylates the NR2A subunit of the NMDA receptor complex and potentiates excitotoxicity mediated by this glutamate-gated ion channel. These data suggest that inhibition of Cdk5 may reduce ischemic injuries for patients.

The present study has not only provided a novel mechanism for ischemia-induced neurodegeneration, but has also pointed to new directions for AD research. Cdk5 activity has been found to be elevated in AD brains (1,2). Mechanisms leading to Cdk5 activation are unclear. Despite inconsistent findings from different laboratories (3-6), an activator of Cdk5, p25, has been reported to be present at higher levels in AD compared to control brains (2,7). The protease responsible for p25 production, calpain, is activated in AD brains as well (6,8,9). Taking these data together, the present paper by Wang et al. would suggest that phosphorylation of the NR2A subunit on Ser1232 might be increased in AD brains. Therefore, examination of the...  Read more

Wang et al. suggest that ischemia-induced neuronal cell death may be mediated by activation of Cdk5. The authors propose that Cdk5 phosphorylates the NR2A subunit of the NMDA receptor complex and potentiates excitotoxicity mediated by this glutamate-gated ion channel. These data suggest that inhibition of Cdk5 may reduce ischemic injuries for patients.

The present study has not only provided a novel mechanism for ischemia-induced neurodegeneration, but has also pointed to new directions for AD research. Cdk5 activity has been found to be elevated in AD brains (1,2). Mechanisms leading to Cdk5 activation are unclear. Despite inconsistent findings from different laboratories (3-6), an activator of Cdk5, p25, has been reported to be present at higher levels in AD compared to control brains (2,7). The protease responsible for p25 production, calpain, is activated in AD brains as well (6,8,9). Taking these data together, the present paper by Wang et al. would suggest that phosphorylation of the NR2A subunit on Ser1232 might be increased in AD brains. Therefore, examination of the phosphoepitope Ser1232 on the NR2A subunit should be highly valuable in further understanding mechanisms behind neurodegeneration in AD. However, protein phosphorylation in general in human brains could be problematic due to potential dephosphorylation during postmortem delay. In addition, since Wang et al. reported changes in NR2A phosphorylation only in the CA1 region but not CA3 and dentate gyrus, a detailed regional analysis of the brain may be warranted. Coincidentally, an NMDA receptor antagonist, memantine, has just been approved by the FDA advisory board for the treatment of moderate to severe forms of AD (see ARF related news story). Whether potential benefits from memantine are derived from its inhibition of Cdk5-enhanced NMDA receptor activity remains to be seen.

References:
1. Lee KY, Clark AW, Rosales JL, Chapman K, Fung T, Johnston RN: Elevated neuronal Cdc2-like kinase activity in the Alzheimer disease brain. Neurosci Res 1999;34(1):21-9. Abstract

2. Patrick GN, Zukerberg L, Mikolic M, de la Monte S, Dikkes P, Tsai L-H: Conversion of p35 to p25 deregulates Cdk5 activity and promotes neurodegeneration. Nature 1999;402:615-22. Abstract

3. Tandon A, Yu H, Wang L, Rogaeva E, Sato C, Chishti MA, Kawarai T, Hasegawa H, Chen F, Davies P, Fraser PE, Westaway D, St George-Hyslop PH: Brain levels of Cdk5 activator p25 are not increased in Alzheimer's or other neurodegenerative diseases with neurofibrillary tangles. J Neurochem 2003;86(3):572-81. Abstract

4. Takashima A, Murayama M, Yasutake K, Takahashi H, Yokoyama M, Ishiguro K. Involvement of cyclin dependent kinase 5 activator p25 on tau phosphorylation in mouse brain. Neurosci Lett 2001;306:37-40. Abstract

5. Yoo BC, Lubec G. p25 protein in neurodegeneration. Nature 2001;411(6839):763-4; discussion 4-5. Abstract

6. Taniguchi S, Fujita Y, Hayashi S, Kakita A, Takahashi H, Murayama S, Saido TC, Hisanaga S, Iwatsubo T, Hasegawa M: Calpain-mediated degradation of p35 to p25 in postmortem human and rat brains. FEBS Lett 2001;489:46-50. Abstract

7. Tseng HC, Zhou Y, Shen Y, Tsai LH: A survey of Cdk5 activator p35 and p25 levels in Alzheimer's disease brains. FEBS Lett 2002;523(1-3):58-62. Abstract

8. Saito K, Elce JS, Hamos JE, Nixon RA: Widespread activation of calcium-activated neutral proteinase (calpain) in the brain in Alzheimer disease: a potential molecular basis for neuronal degeneration. Proc Natl Acad Sci U S A 1993;90(7):2628-32. Abstract

9. Grynspan F, Griffin WR, Cataldo A, Katayama S, Nixon RA: Active site directed antibodies identify calpain II as an early appearing and pervasive component of neurofibrillary pathology in Alzheimer's disease. Brain Res 1997;763:145-58. Abstract

View all comments by Lit-Fui Lau

A previous study from Dr. YouMing Lu and colleagues reported that Cdk5 activation after ischemia leads to activation of the NMDA receptor via Ser1232 phosphorylation of NR2A (Wang et al., 2003). The authors further demonstrated that calpain activation and p25 production following ischemia was prevented by the AMPA receptor antagonist NBQX. In this study, Lu and colleagues provide additional evidence for the role of AMPA receptors during ischemia. Previously it was suggested that ischemia reduces GluR2 levels in CA1, which indicates that AMPA receptors become more permeable to Ca++ influx. This increased permeability may lead to neuronal death. This is the so-called GluR2 hypothesis (Pellegrini-Giampietro et al., 1997).

This study experimentally addresses the GluR2 hypothesis in ischemia by using Semliki Forest Virus (SFV) to mutagenize synaptic AMPA receptor channels in the hippocampus (Liu et al., 2004). The whole-cell patch-clamp recordings elegantly show that reduced GluR2 expression following ischemia induces Ca++ increases in vulnerable neurons. Furthermore, normally...  Read more

A previous study from Dr. YouMing Lu and colleagues reported that Cdk5 activation after ischemia leads to activation of the NMDA receptor via Ser1232 phosphorylation of NR2A (Wang et al., 2003). The authors further demonstrated that calpain activation and p25 production following ischemia was prevented by the AMPA receptor antagonist NBQX. In this study, Lu and colleagues provide additional evidence for the role of AMPA receptors during ischemia. Previously it was suggested that ischemia reduces GluR2 levels in CA1, which indicates that AMPA receptors become more permeable to Ca++ influx. This increased permeability may lead to neuronal death. This is the so-called GluR2 hypothesis (Pellegrini-Giampietro et al., 1997).

This study experimentally addresses the GluR2 hypothesis in ischemia by using Semliki Forest Virus (SFV) to mutagenize synaptic AMPA receptor channels in the hippocampus (Liu et al., 2004). The whole-cell patch-clamp recordings elegantly show that reduced GluR2 expression following ischemia induces Ca++ increases in vulnerable neurons. Furthermore, normally insensitive granule neurons in the hippocampus are rendered vulnerable to ischemia by expressing functional Ca++ permeable GluR2. This validates the GluR2 hypothesis for ischemia and fully supports the mechanism put forth in the previous paper that Ca++ influx through AMPA receptors leads to calpain activation and, ultimately, p35 to p25 conversion.

Intriguingly, several of these findings are recapitulated in Alzheimer’s disease (AD). First, GluR2 levels are decreased prior to NFT formation (Ikonomovic et al., 1997). Furthermore, calpain activity is increased in AD (Saito et al., 1993; Grynspan et al., 1997). Finally, p25 is increased in AD (Patrick et al., 1999; Tseng et al., 2002). This strongly suggests that the GluR2 hypothesis may also be relevant for AD. Therefore, the players in this pathway are all potential therapeutic targets for AD.

References:
Grynspan et al. Active site-directed antibodies identify calpain II as an early-appearing and pervasive component of neurofibrillary pathology in Alzheimer's disease. Brain Res. 1997 Jul 25;763(2):145-58. Abstract

Ikonomovic et al. The loss of GluR2(3) immunoreactivity precedes neurofibrillary tangle formation in the entorhinal cortex and hippocampus of Alzheimer brains. J Neuropathol Exp Neurol. 1997 Sep;56(9):1018-27. Abstract

Liu et al. Expression of Ca(2+)-Permeable AMPA Receptor Channels Primes Cell Death in Transient Forebrain Ischemia. Neuron. 2004 Jul 8;43(1):43-55. Abstract

Patrick et al. Conversion of p35 to p25 deregulates Cdk5 activity and promotes neurodegeneration. Nature. 1999 Dec 9;402(6762):615-22. Abstract

Pellegrini-Giampietro et al. The GluR2 (GluR-B) hypothesis: Ca(2+)-permeable AMPA receptors in neurological disorders. Trends Neurosci. 1997 Oct;20(10):464-70. Review. Abstract

Saito et al. Widespread activation of calcium-activated neutral proteinase (calpain) in the brain in Alzheimer disease: a potential molecular basis for neuronal degeneration. Proc Natl Acad Sci U S A. 1993 Apr 1;90(7):2628-32. Abstract

Wang et al. Cdk5 activation induces hippocampal CA1 cell death by directly phosphorylating NMDA receptors. Nat Neurosci. 2003 Oct;6(10):1039-47. Epub 2003 Sep 21. Abstract

View all comments by Li-Huei Tsai