Dual roles for an intracellular calcium-signaling pathway in regulating synaptic homeostasis and neuronal excitability Douglas J. Brusich , University of Iowa Follow

[摘要] Neurons are specialized cells that communicate via electrical and chemical signaling. It is well-known that homeostatic mechanisms exist to potentiate neuronal output when activity falls. Likewise, while neurons rely on excitable states to function, these same excitable states must be kept in check for stable function. However, the identity of molecular factors and pathways regulating these pathways remain elusive. Chapter 2 of this thesis reports the findings from an RNA interference- and electrophysiology-based screen to identify factors necessary for the long-term maintenance of homeostatic synaptic potentiation. Data is reported to resolve a long-standing question as to the role of presynaptic Ca v 2-type channels in homeostatic synaptic potentiation at the Drosophila NMJ. It is shown that reduction in Ca v 2 channel expression and resultant activity is not sufficient to occlude homeostatic potentiation. Thus, the homeostatic block of a amino-acid substituted Ca v 2-type calcium channel (cac S ) channel is presumed to be due to loss of a specific signaling or binding activity, but not due to overall diminishment in channel function. It is also reported that both Drosophila homologs of phospholipase Cβ (PLCβ) and its putative activator Gαq were found to be necessary for a scaling up of neurotransmitter release upon genetic ablation of glutamate receptors. These factors are canonically involved in the activation of intracellular calcium stores through the inositol trisphosphate receptor (IP 3 R) and the closely related ryanodine receptor (RyR). Likewise, the Drosophila homolog of Cysteine String Protein ( Csp ) is identified as important for long-term homeostatic potentiation. CSP has also been reported to be involved in regulation of intracellular calcium. PLCβ, Gαq, and CSP are also known to regulate Ca v 2-type channels directly, and this possibility, as well as others, are discussed as mechanisms underlying their roles in homeostatic potentiation. Chapter 3 of this thesis reports the extended findings from expression of a gain-of-function Ca v 2-type channel. The Ca v 2.1 channel in humans is known to cause a dominant, heritable form of migraine called familial hemiplegic migraine (FHM). Two amino-acid substitutions causative for migraine were cloned into their analogous residues of the Drosophila Ca v 2 homolog. Expression of these migraine-modeled channels gave rise to several forms of hyperexcitability. Hyperexcitability defects included abnormal evoked waveforms, generation of spontaneous action potential-like events, and multi-quantal release. It is shown that these forms of hyperexcitability can be mitigated through targeted down-regulation of the PLCβ-IP 3 R-RyR intracellular signaling pathway. Chapter 4 presents an extended discussion as to the roles for presynaptic calcium channels, PLCβ, and CSP in homeostatic synaptic potentiation, and the mechanism underlying hyperexcitability downstream of gain-of-function Ca v 2-type channels. The proposed model aims to bridge the involvement of the PLCβ pathway in both homeostatic potentiation and neuronal excitability. Last, the implications for these findings on human disease conditions are elucidated.