Molecular and Cellular Mechanisms of Reelin Signaling in the Adult Hippocampus

Publication Year:
2013
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Repository URL:
https://scholarcommons.usf.edu/etd/4848
Author(s):
Trotter, Justin Howard
Tags:
Disabled-1; Interneurons; Memory; Plasticity; Synapse
thesis / dissertation description
The Reelin signaling pathway is critical for neuronal migration during development and the function of excitatory synapses in the adult forebrain. Despite a growing body of evidence implicating impaired Reelin signaling in the pathogenesis of numerous neuropsychiatric and neurodegenerative disorders, including Schizophrenia and Alzheimer's disease, little is known still regarding the specific molecular and cellular mechanisms whereby Reelin signaling modulates the function of synapses to enable normal learning and memory. In this dissertation, we addressed these knowledge gaps by identifying mechanisms of Reelin proteolysis following synaptic potentiation (Chapter 2) and dissociated the synaptic function of Reelin signaling at excitatory (Chapter 3) and inhibitory synapses (Chapter 4). In the adult brain, Reelin is secreted by GABAergic interneurons into the extracellular space, after which it is cleaved by unknown proteases to generate active fragments that signal downstream. In Chapter 1, we demonstrate that tissue plasminogen activator (tPA) and its major in vivo substrate, plasminogen, cleave Reelin under cell-free conditions to generate major Reelin fragments found in vivo. Since manipulation of tPA levels under basal conditions had no effect on Reelin processing, we hypothesized that synaptic activity may be required to render Reelin susceptible to proteolysis by tPA. Indeed, the modulation of Reelin processing by synaptic potentiation of ex vivo hippocampal slices required the presence of tPA. These data are the first to demonstrate a specific context in which Reelin signaling may be initiated in the intact brain and further emphasize that extracellular proteolysis of Reelin by tPA and other yet-to-be identified proteases is important to consider when trying to understand how altered Reelin processing and/or expression contribute to cognitive impairments associated with disease states. In Chapters 2 and 3 of this dissertation, we describe recent attempts by our lab to elucidate cellular mechanisms of Reelin signaling in the adult brain. To do this, we generated two conditional knockout mutants that lack the obligate downstream adaptor protein, Disabled-1 (Dab1), specifically in postnatal excitatory neurons (eKO) or GABAergic interneurons (iKO). Despite some overlap of Reelin and Dab1 in a subset of GABAergic interneurons, we found that their expression was generally juxtaposed, with Dab1 being primarily expressed by principle neurons and a more widespread population of Reelin-negative GABAergic interneurons. While eKO mice exhibited normal forebrain lamination, dendritic architecture, and dendritic spine density, they did have reductions in spine volume and a loss of basal and activity-dependent Akt and MAPK activation. These changes culminated in impairments in short-term and long-term synaptic plasticity, as well as impairments in associative learning and spatial memory. Taken together, our observations in the eKO mice are the first to definitively establish a synaptic function of Reelin signaling in the adult hippocampus. While characterizing the eKO mice, we also observed that GABAergic interneurons expressed Dab1, which motivated us to explore the inhibitory synapse as a novel locus of Reelin signaling (Chapter 4). Although loss of Dab1 in GABAergic interneurons did not affect forebrain development or the overall patterning of inhibitory synapses, iKO mice presented with an ataxic gait, resting tremor and cerebellar hypoplasia. Interestingly, loss of Dab1 in interneurons led to altered expression of some major glutamatergic synapse proteins (i.e. NMDA receptor subunits NR1 and NR2B), while other excitatory and inhibitory synapse proteins were normal (e.g. NR2A, GAD67/65, and gephyrin). Hippocampal field recordings further demonstrated that even partial loss of Dab1 expression in iHET mice, led to enhanced presynaptic activation and impaired theta-burst induced LTP. These data establish the inhibitory synapse as a novel locus of Reelin signaling in the developing and adult brain. Taken together, data discussed herein should prove useful for understanding and treating disorders associated with Reelin signaling impairments (e.g. AD and Schizophrenia).