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ORCID

http://orcid.org/0000-0002-2553-8494

Date of Award

Spring 5-15-2021

Author's School

Graduate School of Arts and Sciences

Author's Department

Biology & Biomedical Sciences (Neurosciences)

Degree Name

Doctor of Philosophy (PhD)

Degree Type

Dissertation

Abstract

Migraine, an episodic neurological disorder, afflicts about 1 in 10 people at least monthly, yet the underlying pathophysiological mechanisms remain poorly understood. The prototypical monogenic migraine disorder, Familial Hemiplegic Migraine, theoretically presents an excellent opportunity for preclinical modeling, but thus far animal models of migraine have failed to recapitulate the severe migraine aura symptoms of episodic paralysis and ataxia. Mutations that cause Familial Hemiplegic Migraine occur in one of three genes, two neuronal ion channels, and interestingly, the astrocytic α2-Na/K ATPase. In the case of α2-Na/K ATPase, mutations primarily result in loss of protein function. As migraine is thought to be a disease of primarily neuronal hyperexcitability, it is unclear how a loss of α2-Na/K ATPase in astrocytes can confer hyperexcitability non-cell autonomously. To better characterize the sequelae stemming from loss of α2-Na/K ATPase in the brain, we generated α2-Na/K ATPase conditional knockout mice using an astrocyte-selective Cre driver. To our surprise, conditional knockout mice developed episodic paralysis and ataxia not unlike that seen in Familial Hemiplegic Migraine patients. Familial Hemiplegic Migraine motor symptoms present as part of the migraine aura, a set of neurological symptoms associated with a slow wave of synchronized neuronal activity followed by silence known as cortical spreading depression. Using widefield imaging of hemodynamics and neuronal activity via intrinsic optical signal and genetically encoded calcium indicators respectively, we discovered that unanaesthetized conditional knockout mice exhibit cortical spreading depression spontaneously at a similar frequency as paralysis bouts. Moreover, EEG abnormalities known as low voltage activity reliably accompanied cortical spreading depression. By recording continuous bilateral EEG activity in awake and behaving mice, we showed that low voltage activity is coincident with episodes of paralysis and ataxia. In sum, this data suggests that α2-Na/K ATPase conditional knockout mice model Familial Hemiplegic Migraine in an entirely novel way because they develop spontaneous migraine aura and associated motor symptoms. Next, we probed the pathogenic mechanisms of migraine in the conditional knockout mice by performing bulk and astrocyte-enriched RNA-Sequencing on cortex from mice prior to the onset of paralysis symptoms. From these studies, we discovered dysregulated metabolic pathways in the astrocytes, particularly in pathways related to amino acid metabolism. We next performed metabolomics and intersected the bioinformatics data with the metabolite levels. The metabolism network analysis revealed that serine, an uncharged, polar amino acid, is highly upregulated, as are the enzymes and other metabolites closely related to it in the network. Furthermore, HPLC verified that the levels of both enantiomers of serine, D- and L-, are significantly increased. D-serine is a particularly relevant amino acid to neurons because it serves as a co-agonist to the NMDA glutamate receptor, meaning that excess D-serine could theoretically confer neuronal hyperexcitability. We tested the hypothesis that excess serine contributes to the development of migraine aura motor symptoms in conditional knockout mice by implementing a diet with no serine or metabolically related amino acid glycine. Mass spectrometry verified that the serine/glycine free diet substantially lowered the levels of serine in the brain. Strikingly, conditional knockout mice on the serine/glycine free diet take nearly twice as long to develop paralysis symptoms, and symptoms are significantly less severe. Moreover, their motor function is significantly better for the duration of their lives that conditional knockout mice on the control diet. In sum, these studies suggest a novel metabolic pathogenic mechanism in Familial Hemiplegic Migraine. We anticipate that future studies will clarify mechanistic details driving serine buildup after astrocytic α2-Na/K ATPase loss and investigate the therapeutic potential of serine-lowering diets.

Language

English (en)

Chair and Committee

Azad Bonni

Committee Members

Robyn Kleinn, Timothy Miller, Colin Nichols, Guoyan Zhao

Available for download on Sunday, May 15, 2022

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