<p>Intricate neural circuits
underlie all brain functions. However, these neural circuits are highly
dynamic. The ability to change, or the plasticity, of the brain has long been
demonstrated at the level of isolated single synapses under artificial conditions.
Circuit organization and brain function has been extensively studied by
correlating neuronal activity with information input. The primary visual cortex
has become an important model brain region for the study of sensory processing,
in large part due to the ease of manipulating visual stimuli. Much has been
learned from studies of visual cortex focused on understanding the
signal-processing of visual inputs within neural circuits. Many of these
findings are generalizable to other sensory systems and other regions of
cortex. However, few studies have directly demonstrated the orchestrated
neural-circuit plasticity occurring during behavioral experience. </p>
<p>It is vital to
measure the precise circuit connectivity and to quantitatively characterize
experience-dependent circuit plasticity to understand the processes of learning
and memory formation. Moreover, it is important to study how circuit
connectivity and plasticity in neurological and psychiatric disease states
deviates from that in healthy brains. By understanding the impact of disease on
circuit plasticity, it may be possible to develop therapeutic interventions to
alleviate significant neurological and psychiatric morbidity. In the case of
neural trauma or ischemic injury, where neurons and their connections are lost,
functional recovery relies on neural-circuit repair. Evaluating whether neurons
are reconnected into the local circuitry to re-establish the lost connectivity
is crucial for guiding therapeutic development.</p>
<p>There are
several major technical hurdles for studies aiming to quantify circuit
connectivity. First, the lack of high-specificity circuit stimulation methods
and second, the low throughput of the gold-standard patch-clamp technique for
measuring synaptic events have limited progress in this area. To address these
problems, we first engineered the patch-clamp experimental system to automate
the patching process, increasing the throughput and consistency of patch-clamp
electrophysiology while retaining compatibility of the system for experiments
in <i>ex vivo </i>brain slices. We also took
advantage of optogenetics, the technology that enables control of neural
activity with light through ectopic expression of genetically encoded
photo-sensitive channels in targeted neuronal populations. Combining
optogenetic stimulation of pre-synaptic axonal terminals and whole-cell
patch-clamp recording of post-synaptic currents, we mapped the distribution and
strength of synaptic connections from a specific group of neurons onto a single
cell. With the improved patch-clamp efficiency using our automated system, we
efficiently mapped a significant number of neurons in different experimental
conditions/treatments. This approach yielded large datasets, with sufficient
power to make meaningful comparisons between groups.</p>
<p>Using this
method, we first studied visual experience-dependent circuit plasticity in the
primary visual cortex. We measured the connectivity of local feedback and
recurrent neural projections in a Fragile X syndrome mouse model and their
healthy counterparts, with or without a specific visual experience. We found
that repeated visual experience led to increased excitatory drive onto
inhibitory interneurons and intrinsically bursting neurons in healthy animals.
Potentiation at these synapses was absent or abnormal in Fragile X animals.
Furthermore, recurrent excitatory input onto regular spiking neurons within the
same layer remained stable in healthy animals but was depressed in Fragile X
animals following repeated visual experience. These results support the
hypothesis that visual experience leads to selective circuit plasticity which
may underlie the mechanism of visual learning. This circuit plasticity process
is impaired in a mouse model of Fragile X syndrome. </p>
<p>In a separate
study, in collaboration with the laboratory of Dr. Gong Chen, we applied the
circuit-mapping method to measure the effect of a novel brain-repair therapy on
functional circuit recovery following ischemic injury, which locally kills
neurons and creates a glial scar. By directly reprogramming astrocytes into
neurons within the region of the glial scar, this gene-therapy technology aims
to restore the local circuit and thereby dramatically improve behavioral
function after devastating neurological injury. We found that direct
reprogramming converted astrocytes into neurons, and importantly, we found that
these newly reprogrammed neurons integrated appropriately into the local
circuit. The reprogramming also improved connections between surviving endogenous
neurons at the injury site toward normal healthy levels of connectivity.
Connections formed onto the newly reprogrammed neurons spontaneously remodeled,
the process of which resembled neural development. By directly demonstrating
functional connectivity of newly reprogrammed neurons, our results suggest that
this direct reprogramming gene-therapy technology holds significant promise for
future clinical application to restore circuit connectivity and neurological
function following brain injury.</p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/8231852 |
| Date | 02 August 2019 |
| Creators | Qiuyu Wu (6803957) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/MAPPING_BRAIN_CIRCUITS_IN_HEALTH_AND_DISEASE/8231852 |
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