202 H
subunit expression vary with development: GluR2 dow- in synaptic plasticity during brain development, and failure
21
nregulation occur only at those ages at which seizure- to decrease Ca
-permeability of AMPARs at later stages induced damage is observed. These findings provide
may lead to aberrant development.
21
evidence for Ca -permeable AMPARs in the causal chain
Why do neurons which express little or no GluR2 e.g., of events between severe limbic seizures and delayed
hippocampal interneurons and hippocampal pyramidal neuronal death.
neurons of the GluR22 2 knockout mouse [59] survive? Viability of the GluR2 knock-out mice in contrast to that
of the GluR2 editing deficient mice could occur for any of
5. Genetic approaches to the role of GluR2 in a number of reasons. These include 1 compensatory
21
neuronal cell death increases in Ca
buffering and extrusion as, for example,
21
enhanced expression of Ca -binding proteins [68,117],
Gene inactivation knockout and antisense knoc- 2 reduced AMPAR currents, due to slowed receptor
kdown approaches have proven useful in determining the assembly or reduced expression of GluR1 and GluR3,
function of a particular protein under physiological and and or 3 expression of receptors with altered properties,
pathological conditions. Altogether four animal models such as number, localization and interaction of AMPARs
have been developed using genetic techniques: 1 the with signaling and or anchoring proteins and enhanced
Q R editing deficient mouse lacking intron 11 of the desensitization [42]. The viability of these neurons suggest
GluR2 gene; 2 the GluR2 knock-out mouse; 3 the that acute knockdown of GluR2 in neurons that normally
GluR2-flip over expressing mouse; and 4 the gerbil express high levels of GluR2 and relatively rapid changes
21
acutely treated with GluR2 antisense oligonucleotides in AMPAR Ca
permeability may be necessary to induce knockdown. This section briefly reviews the conse-
neuronal death. It should, however, be noted that the quences of these genetic manipulations on brain develop-
GluR2 2 2 knockout was made on a 129 SvEMS3 ment and susceptibility to excitotoxic cell death.
C57BL 6 hybrid, a strain with high resistance to gluta- Heterozygous transgenic mice engineered for a Q R
mate-induced excitotoxicity [42,126]. Moreover, Jia et al. editing deficient GluR2 allele express AMPARs with
1996 [59] did not evaluate the susceptibility of the
21
increased Ca -permeability, particularly in hippocampal
corresponding wild-type mice to excitotoxic cell death. and neocortical principal neurons [16]. The primary conse-
Modifications other than downregulation of GluR2 quence is the onset of spontaneous and recurrent seizures
expression can increase excitotoxicity. The presence of the [16]. The mice develop recurrent seizures and die within
GluR2-flip splice-variant subunit in heteromeric AMPAR the first three weeks of life, with cell loss in the hippocam-
leads to a larger current flow through these channels pus. In these animals, unedited GluR2 may contribute to
[42,89] Transgenic mice that over-express GluR2-flip
21
the formation of a greater number of Ca -permeable
show enhanced susceptibility to excitotoxic glutamate- AMPARs than in the GluR2 knockout mice see below.
mediated damage after permanent middle cerebral artery Transgenic mice with targeted disruption of the GluR2
occlusion, and glutamate excitotoxicity is increased rela- gene ‘GluR2 knock-out mice’ differ considerably from
tive to that of wild-type in neurons cultured from the GluR2Q R editing deficient mice; the knock-out mice are
transgenic mice [72]. Excitotoxicity may be caused by
21 21
viable and fertile. The knock-out mice exhibit a nine-fold increased Ca
influx through voltage-gated Ca chan-
21
increase in kainate-elicited Ca -influx into individual
nels and through NMDAR channels [26,73,84] following CA1 pyramidal neurons and increased inward rectification
increased depolarization mediated by AMPARs containing of kainate-elicited responses and of the AMPAR com-
primarily the flip isoform of GluR2 [42,89]. ponent of EPSCs [59]. The passive membrane properties at
GluR2 knockdown as a technique offers the advantage the resting potential are unchanged, except that input
of examining the effects of GluR2 suppression in an resistance is increased, perhaps due to reduced cell size.
animal that has developed under normal conditions. In- EPSCs are little changed in amplitude and the decay rate is
jection of antisense oligonucleotides directly into the brain unchanged unlike the EPSCs in post ischemic gerbil
of gerbils and rats has been used to demonstrate a probable [146]. The AMPAR-mediated component of the EPSCs is
causal relationship between downregulation of GluR2 reduced relative to the NMDAR-mediated component,
expression and delayed neuronal cell death [100]. This possibly due to reduction in AMPAR density as a result of
study demonstrates that knockdown of GluR2 by intraven- inefficient receptor assembly. LTP in GluR2 knock-out
tricular injection of specific antisense oligonucleotides animals is increased and has a substantial NMDAR-in-
leads to death of CA1 and CA3 neurons. Scrambled dependent component. These data strongly suggest that
antisense administered under the same conditions is with-
21 21
LTP can be mediated by Ca influx through Ca
- out effect. Since the induced neurotoxicity is blocked by
permeable AMPARs see also [46]. The GluR2 knockout 1-naphthyl acetyl spermine, the cause of death is likely to
21 21
animals exhibit significant behavioral changes. Ca -
be Ca influx through GluR2-lacking AMPARs. Further-
permeable AMPARs which are expressed at higher levels more, antisense administered to animals subjected to a
in the postnatal animal than in the adult, may be involved brief ischemic episode which, by itself, causes no neuro-
H . Tanaka et al. Brain Research 886 2000 190 –207
203
nal damage leads to greater cell death than is observed for References
antisense administration to control animals.
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adult rats, Neurosci. Lett. 138 1992 211–215.
future direction would be to apply spatial and temporal
[2] S. Akbarian, M.A. Smith, E.G. Jones, Editing for an AMPA receptor
restrictions to the regulation of GluR2 expression as, for
subunit RNA in prefrontal cortex and striatum in Alzheimer’s
example, by conditional knockout and or knockin ap-
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proaches. Such studies would be expected to aid in our
[3] E.M. Aronica, J.A. Gorter, S. Grooms, J.A. Kessler, M.V. Bennett,
understanding of the role of the GluR2 subunit during
R.S. Zukin, D.M. Rosenbaum, Aurintricarboxylic acid prevents
synaptogenesis and formation of neuronal circuitry and in
GLUR2 mRNA down-regulation and delayed neurodegeneration in
the neurodegeneration associated with stroke and epilepsy.
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[4] C.H. Bailey, P. Montarolo, M. Chen, E.R. Kandel, S. Schacher, Inhibitors of protein and RNA synthesis block structural changes
that accompany long-term heterosynaptic plasticity in Aplysia,
6. Concluding remarks
