Project Summary/Abstract
Activity-dependent associative Hebbian plasticity, is recognized as a core mechanism underlying learning and
memory, cognitive function, and brain development. Hebbian plasticity is intrinsically unstable due to its
positive feedback nature, and has to be balanced by homeostatic mechanisms, which maintain the stability of
overall neuronal activity. A major gap in the field concerns the incomplete understanding of cellular pathways
by which neurons maintain homeostasis and how these mechanisms interact with Hebbian plasticity,
especially on fast time scales. Our preliminary data suggest that a recently discovered neuronal membrane
proteasome (NMP) may be involved in the fast homeostatic mechanism in vivo. NMPs are expressed in the
tadpole brain and degrade nascent proteins in vivo. Inhibition of NMP activity led to a rapid increase in
spontaneous neuronal activity and abolished learning-induced behavioral improvement in a visuomotor
behavior paradigm. Activity-induced de novo synthesis of proteins that are important for the expression of
downstream plasticity mechanisms is a hallmark of Hebbian plasticity. We hypothesize that NMP-mediated
degradation of activity-induced nascent proteins serves as a negative feedback mechanism for fast
homeostatic regulation of neuronal activity in response to plasticity-inducing activities. We will test this
hypothesis in a well-established visually driven experience-dependent plasticity paradigm in Xenopus laevis
tadpoles, which allows the combination of biochemical, physiological, molecular genetics, and behavioral
experiments in an intact neural circuit with physiologically relevant sensory stimulation. Most critically, this
experimental system provides the fast temporal resolution that is pivotal for the investigation of the rapid
degradation of nascent proteins by NMPs in vivo. Specifically, in Aim 1, we will use in vivo BONCAT labeling
to characterize the proteolytic activity of NMPs under different activity regimens and use expansion
microscopy to delineate the spatiotemporal expression profile of NMPs in the optic tectum over development.
In Aim 2, we will combine in vivo Ca++ imaging with molecular genetic tools to examine how NMPs regulate
spontaneous and visually-evoked activity in tectal neurons, and determine whether NMP-mediated regulation
of neuronal activity is cell-autonomous. In Aim3, we will use time-lapse structural and functional imaging and
the visual avoidance behavior to assess the functional role of NMPs in experience-dependent plasticity at
both cellular and circuit levels in the visual system. The proposed experiments will generate data for in-depth
understanding of the NMP function in vivo. These results will shed light on a novel proteostasis-based fast
homeostatic mechanism and lay the groundwork for future studies to further elucidate downstream cellular and
molecular pathways underlying the functional interplay between activity-dependent proteostasis of nascent
proteins and experience- dependent plasticity mechanisms.
