Annalisa Scimemi, PhD - US grants

PROJECT SUMMARY The brain is plastic and is built for changes in its structure and function. Structural and functional plas- ticity have been the subject of intense investigation, but whether similar phenomena occur also in astro- cytes remains enigmatic. The reason for this noticeable gap of knowledge is that until recently, we lacked microscopy approaches that can provide both a high-resolution image of astrocytic processes and a full description of the morphology of entire astrocytes. Expansion microscopy can challenge this status quo by providing an exciting new way to analyze the structure of astrocytes and the distribution of membrane proteins. The overall objective is to determine how the morphology of astrocytes and the sub-cellular distribution of glutamate transporters changes during the sleep/wake cycle. Our central hy- pothesis is that astrocytic processes move away from excitatory synapses during the sleep phase, al- tering glutamate diffusion and weakening excitatory synaptic transmission in the brain. We plan to test our central hypothesis and attain the objective of this application by pursuing the following specific aims: (1) determine the effect of the sleep/wake cycle on astrocyte morphology; (2) map the molecular distribution of glutamate transporters in astrocytes during the sleep/wake cycle; (3) develop 3D models of glutamate diffusion and excitatory transmission during the sleep/wake cycle. The proposed research is significant because it may generate groundbreaking information on how changes in the structure and function of astrocytes contribute to the onset of neurodegenerative diseases. The proposed research is innovative because it aims to develop a new and comprehensive toolbox to investigate fundamental mechanisms regulating the function of the brain. Ultimately, these findings are expected to have an im- portant positive impact in the delineation of the molecular and cellular mechanisms underlying our cog- nitive abilities during health and disease.

Animals have the ability to learn new motor skills and convert them into motor habits. The striatum, a major part of a key motor system in the brain called basal ganglia, is known to exert a fundamental role in controlling the execution of habitual actions. What is not known is exactly how the striatum coordinates the activity of its neurons to ensure proper execution of habitual actions. Recent findings suggest that a particular neurotransmitter transporter, the neuronal glutamate transporter EAAT3, might act as a key player in controlling the time course of excitatory transmission in multiple regions of the brain, including in the striatum. This research project will determine how EAAT3 controls glutamatergic signaling and synaptic integration onto two distinct types of striatal neurons. To accomplish this goal, the investigators use a comprehensive and multidisciplinary toolbox that includes electrophysiological, optical imaging, and computer modeling approaches, as well as mice genetically-engineered that express either D1- or D2-type dopamine neurons.. The project includes training of graduate and undergraduate students in Science, Technology, Engineering, and Mathematics (STEM) disciplines while advancing and transforming our knowledge of striatal circuits. Inclusion of underrepresented minorities and public engagement activities are an integral part of the project, to broaden its impact with the general public.

Our ability to perform movements relies on the activity of a neuronal circuit known as the cortico-striatal-thalamo-cortical (CSTC) pathway. Relay of information in the CSTC pathway relies on the coordinated activation of neurons in the striatum, a major node of the CSTC pathway. The striatum is largely composed of two types of long-projection neurons that express either D1 or D2 dopamine receptors. Glutamate transporters are abundantly expressed in the striatum but their role in controlling the coordinated activity of striatal neurons remains enigmatic. The first goal of this project is to determine how neuronal glutamate transporters regulate excitatory transmission in D1- and D2-expressing neurons. The second goal is to determine how these transporters shape the temporal accuracy with which these cells relay information from incoming excitatory inputs. The experimental strategies entail innovative multi-disciplinary approaches based on electrophysiology, optogenetics, imaging and computer modeling. The proposed research will generate new knowledge on the functional role of glutamate transporters in the activity patterns and dynamics of circuits implicated in habitual movement execution.

In the hippocampus, a brain region involved in learning, memory, and spatial navigation, the ability of synaptic connections between neurons to undergo changes in strength shows circadian rhythmicity. This property of hippocampal synaptic plasticity suggests that the ability to form new memories similarly is regulated dynamically over the course of the day. The daily changes in hippocampal function correlate with cycling levels of hormones, such as melatonin and corticosteroids. This project examines how these hormones contribute to the circadian functionality of neurons, astrocytes, which constitute an important non-neuronal cell type in brain, and excitatory transmission at different sets of hippocampal synapses. Understanding these cellular and molecular mechanisms is crucial for understanding why and how memory formation is modulated across the course of the day. In addition to advancing knowledge of fundamental aspect of hippocampal function, this project provides inter-disciplinary research opportunities for undergraduate students, especially students from under-represented minorities. The project also includes public engagement as a core activity and allows for the trainees? families and friends to learn about the research environment, thereby making a tangible impact on the lives of many first-generation college students.

Numerous studies have identified the molecular underpinnings of different forms of neuronal plasticity, but much less is known about the plasticity of distinct types of glial cells that closely interact with neurons: astrocytes. Astrocytes undergo profound morphological changes in the hypothalamus during lactation, and a growing body of evidence indicates that astrocyte remodeling is much more common than previously thought. Their close proximity to synapses and abundant expression of neurotransmitter transporters allow astrocytes to powerfully control synaptic transmission and plasticity. Yet, there currently is limited knowledge of how these cells modulate excitatory synaptic transmission in the hippocampus, and how their modulation varies across the course of the day. The overall objectives of this project are to: (i) identify the molecular trigger(s) of neuron and astrocyte remodeling; and (ii) determine how these forms of remodeling shape synaptic transmission at different sets of synapses. The central hypothesis is that astrocyte remodeling changes the weight of proximal versus distal excitatory inputs in CA1 pyramidal cells. Identification of the molecular mechanisms that underlie circadian regulation of synaptic transmission and plasticity in the hippocampus and, in particular, the role that astrocytic cells play therein, will advance understanding of the complex mechanisms through which brain activity is modulated by non-neuronal cells.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.