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Molecular biology and genetics provide powerful
tools for understanding and manipulating the brain at the most fundamental
levels of organization. Recent years have seen an explosion of knowledge
about the development of neurons and neural pathways, receptors and
signaling mechanisms, and mechanisms and patterns of gene expression.
These findings are making a profound impact on our understanding of
the nervous system. Furthermore, experimental control of gene expression
allows neural circuits and cognition to be perturbed in a controlled
fashion.
Faculty in the Center use molecular and genetic approaches to identify
cell types, trace functional connections, elucidate molecular mechanisms
of learning and memory, and investigate the proliferation and survival
of neurons in the brain.
Neuronal Cell Types and Their Functional
Connections
Viruses are infectious agents that target
different tissues of the body with great specificity, including
the peripheral and central nervous system. These properties
can be put to use in identifying particular populations of neurons
in the brain and in tracing their synaptic connections. For
instance, pseudorabies virus (PRV) infections are quickly transmitted
through peripheral nerves to sensory ganglia and often to the
brain. This affinity for neurons and ability to spread across
synapses offers a method to map connections between neurons.
PRV is an excellent tracer of the brain's hardwiring, with few
signs of spread to uninvolved circuits.
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Currently,
new tracing viruses are being created by Professor Lynn Enquist that express beta-galactosidase
and variants of green fluorescent protein to easily visualize the
spread of PRV infection in the mammalian nervous system. One such
strain, PRV 152, only infects a few of the visual nuclei that receive
inputs from the retina, including the suprachiasmatic nucleus (SCN)
and the intergeniculate leaflet (IGL) of the thalamus (see figure).
By incubating the infection for different periods of time, the number
of synapses crossed by the virus can be varied. By combining cell-specific
infections with fluorescent labeling, the connections among neuronal
populations can be traced much more efficiently than ever before.
Somewhat surprisingly, physiological tests revealed that infected
cells exhibited completely normal function, indicating that one can
study the functional properties of a neural circuit containing PRV-labeled
cells. Because PRV-infected neurons remain healthy, this virus may
also be engineered to deliver additional genes to specific populations
of cells within the brain and alter the function of that neural circuit.
Molecular Mechanisms of Learning and Memory
Use-dependent modifications of synaptic efficacy,
known as synaptic plasticity, are thought to underlie learning and
memory in adult animals and to be important for fine-tuning the neural
pattern formations in the developing brain. By taking advantage of
newly established gene knockout technology, a series of mouse strains
can be engineered that carry mutations in genes believed to be involved
in neuronal synaptic plasticity. The impact of gene mutations can
then be systematically analyzed at molecular, synaptic, physiological,
and behavioral levels. The long-term goal of this research, conducted by
Professor Joseph Tsien, is to elucidate
the molecular and cellular mechanisms of cognitive behavior such as
learning and memory in the mammalian brain, and to identify potential
molecular targets and/or strategies for the treatment of memory disorders.
Genetic techniques have included the successful
development of a Cre/loxP-mediated knockout of NMDA receptors
that is restricted to the CA1 region of the hippocampus, a brain
structure involved in learning and memory. These mice lack the
ability to enhance synaptic efficacy between neurons in a manner
that can last for hours, days, and even weeks, an ability known
as long- term potentiation (LTP); they also show defects in
a spatial learning task, known as the Morris water maze. More
recently, transgenic mice have been engineered to overexpress
the NR2B subunit of the NMDA receptor; these mice exhibit enhanced
LTP as well as an improved ability to learn and remember.
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To further deepen our understanding of brain
function, we have begun to profile gene expression patterns in the
both developing and adult brains using DNA microarray technology in
order to establish a comprehensive Brain Genome database. In addition,
we are currently exploring a new chemical-genetic approach, which
can be used for the development of the third-generation genetic technology.
By developing new molecular and genetic technologies, we expect to
open new doors in elucidation of the cellular mechanisms of multi-staged
memory processes and in the treatment of learning and memory disorders.
Neuronal Proliferation and Survival
Another ongoing research effort at the
CSBMB, conducted by Professor Elizabeth Gould,
aims to understand the conditions that permit the appropriate
proliferation and survival of neurons in the brain. This information
is important for understanding normal and abnormal brain development,
as well as the inability of the adult brain to replace neurons
lost following trauma or disease. The granule neuron population
of the rat dentate gyrus has been the main focus of these studies
because, unlike the majority of neuronal populations, it is
formed predominantly during the postnatal period, and new granule
neurons continue to be produced well into adulthood. Moreover,
the dentate gyrus has the unique ability to produce neurons
following damage. This system offers the unique opportunity
to explore parallel questions of cell proliferation and survival
in the intact brain during development and in adulthood. Current
topics of research include the following: Hormonal and neurotransmitter
factors that regulate cell proliferation and survival; experience-dependent
changes in neurogenesis; functional consequences of changes
in the rate of neurogenesis;and
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Figure 3 |
clinical implications, including the possibility
that the unique plasticity of the granule neuron population is partially
responsible for the unusual resistance of the dentate gyrus to degeneration
in pathological human conditions such as epilepsy and Alzheimer's
disease.
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