Dissertations, Theses, and Capstone Projects

Date of Degree

2-2017

Document Type

Dissertation

Degree Name

Ph.D.

Program

Biology

Advisor

Mitchell Goldfarb

Committee Members

Jayne Raper

Marom Bikson

John Koester

Henry Colecraft

Subject Categories

Biology | Molecular and Cellular Neuroscience

Keywords

Channelopathies, Early-onset epileptic encephalopathy (EOEE), Neurological disorders, Spike frequency accommodation

Abstract

Purpose: FHFs are cytosolic proteins that bind to voltage-gated sodium channels (NaVs) and modulate their functions to control membrane excitability. FHFs raise the voltage dependence of NaV fast inactivation to promote excitability, while A isoforms FHF (A-FHFs) also capture open sodium channels into a long-term inactivated (LTI) state to limit excitability. My research has addressed how FHFs balance membrane excitability as it relates to normal and pathological brain functions. Part of this work stemmed from the discovery of a missense FHF1 mutation in patients with severe early onset epilepsy. Methods and Results: Wild-type FHF1A and FHF1B were compared to derivative proteins bearing the epilepsy-associated mutation for their ability to raise the voltage dependence of NaV inactivation. We found that the epilepsy missense FHF1 mutation is gain-of-function, enabling aberrant FHF1 isoforms to further elevate the voltage at which sodium channels inactivate. These findings offer a clear rationale for how the mutation promotes epilepsy. To investigate the physical mechanism and biological consequences of A-FHF-mediated NaV LTI, we did functional testing of A-FHF proteins bearing amino acid substitutions along with microinjection of an antibody that specifically blocks A-FHF-mediated NaV LTI. We found that A-FHF proteins bear an N-terminal motif that employs cationic and aliphatic residues to induce and maintain the NaV long-term inactivated state. Furthermore, antibody blockade of NaV LTI mediated by endogenous A-FHFs in hippocampal pyramidal neurons enhances neural excitability by suppressing spike frequency accommodation. Conclusions: FHFs can naturally promote or limit neuronal excitability. When the pro-excitatory function of FHF1 proteins is enhanced by mutation, epilepsy could be triggered. Neuronal excitability can also be enhanced if A-FHF mediated NaV LTI is suppressed.

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