2023 Theses Doctoral
Deciphering axon dysfunction in the pathogenesis of ARHGEF9 epileptic encephalopathy
Developmental and epileptic encephalopathies (DEE) represent a set of rare but devastating and largely intractable childhood epilepsies. While mouse models have made innumerable contributions to understanding the genetic basis of neurological diseases, only a small fraction of missense, gain-of-function DEE variants has been modeled in mice. In this dissertation, we focus on one DEE gene, ARHGEF9. With fewer than fifty ARHGEF9 patients reported to date, ARHGEF9 can be considered one of the rare players in DEE. ARHGEF9 encodes a brain-specific protein also known as collybistin (CB), a guanine nucleotide exchange factor and an essential regulator of inhibitory postsynaptic density.
In Chapter 3 and Chapter 4, we present results and ongoing studies from our efforts to unravel the pathological mechanism of ARHGEF9 DEE. We studied the G55A variant on the SH3 domain, which was discovered in one severe case of DEE. Using a novel Arhgef9G55A mouse model, we examined behavioral, cellular, and electrophysiological consequences of Arhgef9G55A. Results demonstrate that the Arhgef9G55A mouse model is an adequate ARHGEF9 DEE model, because it phenocopies key aspects of human ARHGEF9 DEE. We showed the interesting protein aggregation phenotype caused by the G55A variant. Specifically, in Arhgef9G55A/Y neurons, CB forms protein aggregates at the proximal AIS, leading to dramatic disruptions in inhibitory postsynaptic components at the AIS. Furthermore, electrophysiological studies revealed significant changes in intrinsic neuronal excitability and synaptic transmission in Arhgef9G55A/Y brains. The work within this dissertation shows that the G55A variant disrupts axon initial segment structure and functions.
In Chapter 2, we review and summarize current understandings on AIS structure and functions. We highlight the central role of the AIS in initiating action potential and integrating synaptic inputs through axo-axonic synapses. Based on our experimental results, we propose that disruptions in AIS function are closely tied to the pathophysiology of ARHGEF9 DEE. Aside from the clinical significance of our study, we demonstrate the important role of CB at the AIS. We propose that CB is a specific stabilizer of axo-axonic synapses. The difference in the requirement of CB in inhibitory synapse formation in different neuronal compartments could be a core molecular machinery underlying the functional diversity of inhibitory inputs.
This item is currently under embargo. It will be available starting 2024-11-16.
More About This Work
- Academic Units
- Genetics and Development
- Thesis Advisors
- Frankel, Wayne
- Ph.D., Columbia University
- Published Here
- November 23, 2022