Author's School

Graduate School of Arts & Sciences

Author's Department/Program

Biology and Biomedical Sciences: Neurosciences

Language

English (en)

Date of Award

8-13-2012

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Chair and Committee

Steven Mennerick

Abstract

Neurons are responsible for information processing within the nervous system, so strong perturbations of neuronal function have far-reaching consequences within the neural network. Damage in response to excess excitation, as occurs during stroke or seizure, is known as excitotoxicity. One method utilized by neurons for reducing excitotoxicity within an overly activated neuronal network is to arrest excitatory neurotransmitter release from presynaptic terminals. The mechanisms responsible for inducing this presynaptic silencing: or "muting"), however, have been elusive. In order to elucidate the signals responsible, I used molecular techniques in defined networks of cultured neurons from the mammalian hippocampus, a well-studied brain region known to be important for learning and memory but susceptible to excitotoxic damage. Calcium serves as a signal transducer during excitotoxicity and many forms of synaptic plasticity, but the signaling cascades in presynaptic silencing were previously unknown. In neurons individually depolarized via heterologous ion channel activation, I showed that calcium influx led to cell death while channel expression led to synaptic depression, although muting was not confirmed. Calcium, however, was not necessary for presynaptic muting after strong depolarization. Instead, inhibitory G-protein signaling induced silencing through cyclic adenosine monophosphate: cAMP) reduction but surprisingly not via activation of likely candidate receptors. This cAMP reduction contributed to loss of proteins important for vesicle fusion at the presynaptic terminal. I also found that astrocytes, support cells in the nervous system that have garnered attention recently for their ability to modulate neuronal function, were required for the proper development of presynaptic muting in hippocampal neurons. Soluble factors released by astrocytes were permissive, but not instructive, for silencing induction. Thrombospondins were identified as the astrocyte-derived factors responsible for muting competence in neurons, and they act through binding to the a2d-1 subunit of voltage-gated calcium channels. cAMP-activated protein kinase A exhibited dysfunctional behavior in the absence of thrombospondins, potentially explaining the presynaptic muting deficit in an astrocyte-deficient environment. Together these results clarify the molecular mechanisms responsible for an underappreciated form of neuroprotective synaptic plasticity and provide potential therapeutic targets for a number of disorders expressing excitotoxic damage.

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Permanent URL: http://dx.doi.org/10.7936/K7222RSQ

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