Research Summary:
I am examining
the cytochemical characteristics of axospinous synapses in hippocampal
CA1 stratum radiatum with immunogold electron microscopy for AMPA
receptors (AMPARs) and NMDA receptors (NMDARs). Neurotransmitter receptors
for glutamate are a major protein constituent of the postsynaptic
density (PSD) in CA1 stratum radiatum (Wheal et al., 1998). AMPARs
mediate the majority of fast excitatory synaptic transmission, and
the calcium influx through NMDARs is an important regulator of signal
transduction cascades. Together, these two receptors activate endosomal
and exocytotic processes that traffic AMPARs into and out of the postsynaptic
membrane (Ehlers, 2000). The number/density of AMPARs in the PSD is
the major determinant of synaptic efficacy (Malenka & Nicoll,
1999; Luscher & Frerking, 2002), implying that the cytochemistry
of hippocampal synapses determines their strength.
In collaboration with Yuri Geinisman, I am examining the linkage between
synaptic cytochemistry and synaptic efficacy in two main ways. First,
it is well-established that synapses on the distal branches of CA1
pyramidal neurons compensate for their distance from the soma by generating
larger ionic currents than synapses located more proximally (Magee,
2000). The distant-dependent compensation in CA1 stratum radiatum
synapses could be detected with electron microscopy in three main
ways: i) larger PSDs, and by extension more synaptic receptors, in
distal synapses; ii) higher AMPAR immunoreactivity in distal synapses;
and iii) distal dendritic synapses being represented by a disproportionately
high number of perforated synapses, which previous studies have shown
have an unusually high number and concentration of AMPARs and NMDARs
(Ganeshina et al., 2004a; 2004b). And second, a subset of aged rats
can learn as well as young adults, whereas other rats the same exact
age show severe hippocampus-dependent learning impairments (Barnes,
1994; Gallagher & Rapp, 1997; Knuttinen et al., 2001). We showed
that the PSD size of perforated synapses in CA1 stratum radiatum is
selectively reduced in the subset of aged rats exhibiting impairments
in the Morris water maze (Nicholson et al., 2004), a spatial learning
task that requires the hippocampus. Because perforated synapses have
an extraordinarily high number of AMPARs, the size reduction in their
PSD could be attributable to deAMPAfication. We are currently testing
this hypothesis with aged rats that have been behaviorally characterized
using trace eyeblink conditioning, another hippocampus-dependent learning
task (Moyer et al., 1990).
An important avenue of future research will be to pinpoint the cellular
processes responsible for the PSD size reduction, and to determine
whether they are also involved in learning-related synaptic plasticity.
For example, is the age-related deAMPAfication of perforated synapses
a failure to reinsert endocytosed AMPARs (an NMDA, Ca2+, protein phosphatase,
and protein kinase A-dependent process), or is it a product of unregulated
lysosomal/proteasomal degradation? Alternatively, newly synthesized
AMPARs are secreted through the endoplasmic reticulum/Golgi apparatus
system (Horton & Ehlers, 2003; Vandenberghe & Bredt, 2004),
and then mobilized to the postsynaptic membrane by transport vesicles
(Ehlers, 2000). Any change in this fundamental cellular process will
disrupt a neuron's ability to replace degraded synaptic proteins,
and by default cause widespread deAMPAfication. AMPARs and NMDARs
are anchored to the PSD by scaffolding proteins that contain PDZ domains
(Sheng & Sala, 2001), and the scaffolding proteins connect the
PSD to the actin cytoskeleton (Matus, 2000). Does deAMPAfication precede,
follow, or coincide with actin-based cytoskeletal rearrangements?
Moreover, do any of these processes vary with distance from the soma?
By combining electron microscopy and immunocytochemistry, I hope to
help elucidate some of the mechanisms of age-related hippocampal dysfunction
and synaptic plasticity and, in the end, provide fundamental insights
into their function and consequences for learning and memory.To review
my CV please click
here.
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Last updated: Sunday, July 28, 2002
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