2012 — 2016 |
Eroglu, Cagla |
R01Activity Code Description: To support a discrete, specified, circumscribed project to be performed by the named investigator(s) in an area representing his or her specific interest and competencies. |
Control of Synapse Formation and Maturation by Astrocytes
DESCRIPTION (provided by applicant): Synapses are the fundamental functional units of the nervous system, but the molecular and cellular interactions that regulate their establishment are largely unknown. Studies using purified retinal ganglion cell neurons (RGCs) showed that astrocytes secrete signals such as thrombospondins that strongly induce excitatory synapse formation. In our preliminary studies, we identified another astrocyte-secreted synaptogenic protein, hevin. Addition of hevin to purified RGC cultures robustly stimulates excitatory synaptogenesis. Moreover, Hevin-null mice have significantly less excitatory synapses that present striking structural defects suggesting that hevin is required for the formation and morphological maturation of synapses in vivo. Astrocytes also express a close homolog of hevin called SPARC. Intriguingly, SPARC is not synaptogenic but specifically inhibits hevin-induced synaptogenesis. These data show for the first time that astrocytes regulate synaptic connectivity not only by stimulating, but also by inhibiting synaptogenesis. Our results signify the exciting possibility that astrocytes, through the regulation of relative levels of hevin and SPARC, can actively control the development and function of synaptic networks in the developing and adult brain. How hevin induces synapse formation and the nature of SPARC's antagonistic function are unknown. Therefore, our objective here is to unravel a novel molecular mechanism of regulation of synaptic development and maintenance by astrocytes through hevin/SPARC signaling. In this application, we will first test the hypothesis that hevin and SPARC regulate synaptic morphology (Aim 1) and formation of dendritic spine synapses in vivo (Aim 2). Second, we will determine the contribution of hevin/SPARC signaling to synaptic function in vivo (Aim 2). Third, we will test the hypothesis that hevin mediates synaptogenesis through interactions with the trans-synaptic adhesion molecules neurexins and neuroligins, whereas SPARC antagonizes hevin by competing for hevin-binding to neuroligins (Aim 3). These studies are important since they will provide new insights into the control of formation, maintenance and function of synapses by astrocytes. These new insights will have a significant positive impact by advancing our molecular and cellular understanding of astrocyte-neuron interactions that orchestrate central nervous system development and function. A deeper mechanistic understanding of synapse formation and how astrocytes participate in this process will lead to the development of innovative approaches to prevent or cure neurological disorders such as autism, depression and addiction.
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1 |
2016 — 2020 |
Eroglu, Cagla |
R01Activity Code Description: To support a discrete, specified, circumscribed project to be performed by the named investigator(s) in an area representing his or her specific interest and competencies. |
The Regulation of Synaptic Connectivity and Homeostasis by Huntingtin
? DESCRIPTION (provided by applicant): Huntington's disease (HD) is a fatal neurodegenerative disease caused by mutations introducing an extended stretch of poly-glutamines (poly-Q) at the N-terminus of huntingtin (Htt). A widely accepted, yet unproven, hypothesis is that HD is caused by gain-of-function, toxic effects of mutant Htt protein. In recent years, dominant negative loss-of-function effects of poly-Q mutations have also emerged as drivers of disease pathophysiology. However, despite what is known about pathophysiology of mutant Htt, the functions of wildtype (WT) Htt are still largely unknown. Astrocytes, the major glial cells of the brain, secrete synaptogenic thrombospondin family proteins to initiate synapse formation. Thrombospondin induces synaptogenesis by binding to a neuronal receptor, the gabapentin receptor ?2?-1. In our preliminary experiments, we found that ?2?-1 interacts with huntingtin and this interaction is impaired in the presence of poly-Q expansions. Early synaptic problems in the excitatory cortical and striatal connections have been reported in HD, but whether huntingtin played a role in synaptic connectivity was unknown. By conditionally silencing Htt in the mouse cortex we showed that huntingtin controls synapse formation and maturation within cortical and striatal circuits. Moreover, by using an HD mouse model, we found that this function of huntingtin is lost when the pathogenic poly-glutamine mutation is present. Based on these findings, here we will test the hypotheses that huntingtin controls synaptic connectivity through its interaction with ?2?-1, and that the impairment of this interaction in the presence of the disease-causing poly-Q mutations leads to detrimental errors in synaptic connectivity. The loss-of-function effects of mutant Htt during development may be important for driving the disease onset and could underlie prodromal neurological symptoms of HD. Therefore, understanding the function of WT Htt in synaptic development may enable us to find ways to correct the developmental errors in the cortical and striatal circuits of mutant Huntingtin carriers. This approach could therefore lead to the prevention of disease onset or greatly diminished disease progression, allowing HD patients to live full, healthy lives.
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1 |
2018 — 2020 |
Eroglu, Cagla |
R01Activity Code Description: To support a discrete, specified, circumscribed project to be performed by the named investigator(s) in an area representing his or her specific interest and competencies. |
Control of Astrocyte Development and Astrocyte-Synapse Interactions
Astrocytes are highly complex cells with hundreds of thousands of fine processes that contact and ensheathe neuronal synapses. These perisynaptic astrocyte processes actively participate in synaptic development and function by regulating neurotransmitter release, maintaining ion homeostasis, and modulating synaptic connectivity. Despite the importance of astrocytes in synaptic development and function, we know very little about the molecular and cellular mechanisms that control the establishment of complex astrocyte morphology and astrocyte-synapse interactions. In our preliminary experiments, we found that in the mouse visual cortex the establishment of the complex astrocyte morphology is a developmentally regulated process that occurs during a period of extensive synapse formation. Manipulation of visual experience, i.e. dark rearing mice during first three weeks of postnatal development, strongly stunts cortical astrocyte development, indicating that experience- dependent changes in synaptic connectivity can alter morphological maturation of astrocytes. How is the complex astrocyte morphology attained and remodeled? To mechanistically address this question, we developed a primary cortical neuron and astrocyte co-culture system that takes advantage of the following basic observation: Astrocytes cultured by themselves have a simple fibroblast-like morphology; however, contact with neurons, even for a short period of time, is sufficient to trigger extensive elaboration of the astrocytes. This morphological shift in astrocytes is primarily driven by direct neuronal contact, but not by soluble secreted factors. Using this system, we conducted a candidate-based genetic screen and identified that the astrocytic expression of neuroligin (NL) family cell-adhesion molecules (CAMs), NL1, NL2 and NL3 control neuronal adherence and morphological maturation of astrocytes in vitro and in vivo. Based on these findings, our objective here is to determine the functions of NLs in astrocyte development and astrocyte- synapse interactions. To do so, we will test three hypotheses: 1) Astrocytic NLs control astrocyte morphological complexity by mediating astrocyte-synapse association. 2) NLs perform these functions via their extracellular interactions with axonal/presynaptic neurexins and 3) via their critical intracellular domains that control cytoskeletal dynamics within astrocytes. These studies have the potential to significantly advance our understanding of the molecular basis of astrocyte development and astrocyte-synapse interactions. Moreover, the results obtained here will provide critical new avenues for studying the formation of the tripartite synapses and reveal a new paradigm through which astrocytic NLs control brain development, a process that may be critically impaired in neurological disorders.!
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1 |
2019 — 2020 |
Eroglu, Cagla Soderling, Scott H. [⬀] |
R01Activity Code Description: To support a discrete, specified, circumscribed project to be performed by the named investigator(s) in an area representing his or her specific interest and competencies. |
New Proteomic and Genome Engineering Approaches to Decipher Astrocyte Function At Synapses
ABSTRACT Astrocytes are the most abundant glial cells in the human brain. Interactions of astrocytes with synapses via thin perisynaptic astrocytic processes are critical for proper synaptic connectivity and function. Each mouse astrocyte sends out an extensive array of processes that are estimated to contact over 100,000 synapses. The number of astrocytes and the extent of their interactions with synapses have increased throughout evolution, indicating a close link between astrocytes and cognition. Moreover, emerging evidence suggests that dysfunction of astrocyte-synapse interactions contributes to a variety of brain disorders. In contrast to neuronal synaptic structures, however, we are largely blind to the molecular composition and mechanisms of the astrocytic perisynaptic structures. Moreover, there is currently very little understanding of how mutations that disrupt astrocyte-synapse interactions lead to synaptic pathologies. This is in large part because, unlike neuronal synaptic structures, it has not been possible to purify and identify proteins enriched at subcellular regions of astrocytes. In this project, we will develop and utilize innovative proteomic and genome editing approaches to solve these problems, revealing the proteins and inner workings of astrocyte processes that associate with and modulate synapses. We anticipate these data will provide a new and unparalleled molecular framework for future studies on astrocyte- neuron interactions.
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1 |