One of the hallmarks of long-term memory storage is that
it requires the synthesis of new genes and new proteins, which
act to alter the strength of synaptic connections within appropriate
neuronal circuits in the brain. Indeed, experiments first carried
out over forty years ago have shown that protein synthesis and
transcription inhibitors selectively block long-term memory in
a variety of organisms and a number of behavioral tasks. The
challenge has been to define the molecular mechanisms by which
gene regulation occurs and to identify how these biochemical
changes act within specific neural circuits to alter behavior.
Initial work examined the role of individual transcription factors,
with a focus on those transcriptional regulatory proteins that
are modified by cellular signaling pathways activated by neuronal
activity and neurotransmitter receptors.
Structural magnetic resonance imaging (MRI) is used to
image brain morphology in genetically modified mice. The
hippocampus (magenta) and amygdala (blue) are highlighted
in this image.
What are the mechanisms
of transcriptional regulation during memory storage?
How are the various signals acting on a neuron integrated to
give rise to appropriate changes in gene expression? How are
changes in gene expression maintained to sustain memories for
days, months and even years? Our work has revealed that transcriptional
coactivators, such as CREB-binding protein (CBP) represent
the critical molecular switches that integrate signals via
multiple
signaling pathways and multiple transcriptional regulatory
proteins. Further, CBP is a histone acetyltransferase capable
of the epigenetic
modification of chromatin. These epigenetic marks act to stable
alter the expression of specific sets of genes during the consolidation
of long-term memory. We have been taking a genetic approach
to define the role of CBP and a related coactivator p300 in
long-term
memory storage. Our work has revealed long-term memory deficits
in genetically modified mice in which CBP function has been
impaired by transgenic expression of an inhibitory form of
CBP or in mice
that carry a mutant form of CBP unable to be recruited to DNA
by the transcription factor CREB. Using pharmacological approaches,
we have transiently increased histone acetylation in the hippocampus
during memory consolidation using HDAC inhibitors. This pharmacological
increase in histone acetylation results in enhanced memory
storage and increased synaptic strength. Thus, our work suggests
that
epigenetic mechanisms of chromatin modifications mediate the
storage of memory in neuronal circuits within the brain. A
critical question now is to identify the genes whose expression
is coordinately
regulated by epigenetic during the consolidation of long-term
memory. These experiments are currently underway using quantitative
PCR, microarray analysis and chromatin immunoprecipitation.
Initial observations have revealed a surprisingly specific
regulation
of a subset of genes by histone acetylation, including two
members of the nuclear hormone receptor superfamily. These
nuclear hormone
receptors may mediate a cascade of gene expression that serves
to stably maintain long-term memories on days, weeks and even
years.
Where is the cAMP/PKA signaling pathway activated within
neurons during memory storage?
One of the challenges in the study of signal transduction
pathways in neurons is how the synapse specificity of synaptic
plasticity
is maintained in the face of diffusible second messengers,
such as cAMP, and diffusible proteins, such as the catalytic
subunit
of PKA. This is especially true for long-lasting forms of
synaptic plasticity and long-term memory storage that involve
changes
in gene expression, for there is only a single nucleus even
though a neuron may have thousands of individual synapses.
One way in
which neurons solve this problem is to restrict PKA to specific
subcellular locations via interactions between PKA and A-kinase
anchoring proteins (AKAPs). Therefore, we are addressing
the role of PKA anchoring in hippocampal synaptic plasticity
and
memory storage using genetic and pharmacological approaches.
Transgenic mice expressing a fluorescently tagged regulatory
subunit of protein kinase A allow us to study the localization
of this kinase in neurons in the hippocampus.
Our initial studies have revealed that PKA activity and PKA anchoring
is required for synaptic tagging, the process by which plasticity-related
proteins are “captured” by activated synapses.
We are now investigating which AKAP signaling complexes are
crucial for these processes and defining the behavioral role
of PKA anchoring. This analysis will be extended to identify
the substrates of PKA targeted by anchored kinase, enabling
us to determine the pool of AKAP complexes and PKA substrates
that are required for synaptic tagging and capture.
To address the critical question of where, when and how signal
transduction pathways are activated in neurons, we have developed
novel techniques to measure cAMP levels and PKA activity in discrete
subcellular compartments of neurons in vivo.
By optically detecting
PKA localization and activation in living neurons from transgenic
mice expressing fluorescently labeled
PKA subunits, we are visualizing the spatiotemporal patterns
of cAMP signals following the induction of synaptic potentiation.
In addition to visualizing cAMP microdomains in neurons,
we will use this approach to examine the idea that the catalytic
subunit of PKA translocates to the nucleus to activate gene
expression during long-lasting forms of synaptic plasticity.
This research promises to reveal how signal transduction
pathways
are modulated by the subcellular localization of kinases
and to elucidate the spatiotemporal activation patterns of neuronal
signal transduction pathways, providing insight into the
biochemical
mechanisms employed by neurons to store information.
How does
sleep modulate memory storage?
The biological function of sleep has remained elusive, but
studies suggest that one function of sleep may be to mediate
memory storage.
We have recently begun to use genetic, electrophysiological and
behavioral approaches to understand the regulation of sleep and
the role of sleep in memory storage—with interesting findings.
First, sleep appears to be critical for the storage of hippocampus-dependent
memories, and sleep is increased following training. Second,
sleep appears to be regulated by many of the same molecular processes
that regulate memory storage, including the transcription factor
CREB and the PKA signaling pathway. By using conditional genetic
approaches and microarray studies, we will elucidate the machinery
underlying sleep and define the role of sleep in the consolidation
of long-term memory. Thus, our genetic approaches to study the
role of specific signal transduction pathways in neuronal function
may ultimately lead to an understanding not only of the mechanisms
of memory storage, but also the function of sleep, providing
insight into how the processes of memory and sleep are disrupted
by aging as well as neurological and psychiatric disorders.
selected
publications
Huang, T., McDonough, C. and Abel, T. (2006). Compartmentalized
PKA signaling events are required for synaptic tagging and capture
during hippocampal LTP. European Journal of Cell Biology 85:
635-642.
Wood, M. A., Kaplan, M. P., Park, A., Blanchard, E. J., Oliveira,
A. M. M., Lombardi, T. L. and Abel, T. (2005). Transgenic mice
expressing a truncated form of CREB-binding protein (CBP) exhibit
deficits in hippocampal synaptic plasticity and memory storage.
Learning & Memory 12: 111-119.
Lattal, K. M. and Abel, T. (2004). Behavioral impairments caused
by injections of the protein synthesis inhibitor anisomycin after
contextual retrieval reverse with time. Proceedings of the
National Academy of Sciences 101: 4667-4672.
Graves, L., Heller, E., Pack, A. and Abel, T. (2003). Sleep
deprivation selectively impairs memory consolidation for contextual
fear conditioning. Learning & Memory 10: 168-176.
Bucan, M. and Abel, T. (2002). The mouse: Genetics meets behavior.
Nature Reviews Genetics 3:114-123.
Abel, T. and Lattal, K. M. (2001). Molecular mechanisms of memory
acquisition, consolidation and retrieval. Current Opinion
in Neurobiology 11: 180-187.
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